Soxfan963,
Twisted cables.
Example.RJ45 cables have every 2 twisted, so out of 8 wires there are 4 twisted pair.
Cables in the Controller that connect to the motors should be twisted pair.
Cable going to the motors should be twisted pair.
Homing cables should also be twisted pair.
The power cable if you are using a VFD HS spindle the power cable should have a shield with one end earthed.
A shielded cables may have a foil surrounding the twisted cables or it could have a wire mesh.
This is a link to some shielded and twisted cables.
https://www.homanndesigns.com/index.php?main_page=index&cPath=55_11&zenid=e8tv3cq1e8f5r0mepvapes6ve3
Hope this helps.
Regards,
Mauri.

Soxfan963,

If I run the machine at a VERY slow feed rate, it runs fine. only as I speed it up do I run into problems....  

Your router is an emitter source of EMI. Anything that sparks creates noise,
electromagnetic energy noise ( signals we can't understand ). The faster it runs the more noise it is creating. The electromagnetic energy noise could be envisioned as a ball / field  of dirty air around the emitter and the intensity of the dirty air drops off relative to distance. Anything, wire, control box, computer,that cuts that field will couple / have a voltage created on it and the
offending voltage, frequency dependent in many cases, may interfere with a device causing interference to it.

Dealing with EMI is problematic because one needs to have understanding of what is happening, able to look at the big picture,and have a good understanding of basics
relative to RF / EMI. There are "books" written on the subject. I say this because solutions to a problem usually are just some partial suggestions / narrow view of the problem on how to deal with it.
 
Here is some general info at a high level:

EMI / RF / noise creation requires a source of creation which could be called an emitter. The noise can be anything from specific to broad spectrum of frequencies
and vary in intensity and duration. There must be a path from the emitter to receiver and provide for inductive, conductive, or radiated coupling.Then there must be a reciever of the noise. One can address the problem by eliminating the source ( or any one of the above three), minimizing the intensity, protecting the recievers, providing different levels of grounding. By grounding, I mean in terms of providing an less restrictive / independant path to ground for the noise.      

Now I must say  even the most experienced folks on the subject will comment,
" sometimes it's like black magic".

Just some insight on the matter,

RICH

As a quick and dirty test

Air cut the program with the spindle off if it still stops the problem is elsewhere

Now this is not to say the spindle is totally in the clear

You could have more than one mode of failure rather than a common mode one

It is always worth while to Earth the chassis and any enclosures ( equiptential bonding ) this can help

It is a real pain to sort these problems out but you must be methodical change only one thing and test and so on

Thanks for your reply Rich. I appreciate your knowledge of this subject. I think I understand the basics of what you're getting at here and I'm still working my way through the diagnosis of my problem. 

Here's where I'm at currently...

-The machine works as expected while "Air cutting" as Stuart calls it at every reasonable speed (I havent tested past 25FPM as I can't see ever running a program that fast).
-I can cut a part out of MDF at about 10FPM with my spindle speed at about 24,000 (the dewalt router that I have mounted in there is 16k to 27k RPM so that's about 4.5 out of 6 on the variable speed dial.) The cut sounds smooth at a depth of 3/16-1/4" with a 1/4" spiral mill)
-In hardwoods like birch or maple, (same cutter/depth of cut) the speed can't exceed 8fpm and the spindle speed needs to slow down a little more (around 20k-22k rpm - maybe a 4 on the router dial). if while it's cutting I increase the spindle speed it will stop.

So this gets me wondering... what are common feed rates/cutting depths? Obviously it is dependent on wood density, bit size/type, sharpness etc, but what's reasonable?

Let's take this scenario:
Hard maple, Brand new 1/4" up spiral flat end mill. What's your feed rate, spindle speed, and depth of cut?

Soxfan963,
You do not say if you are using HSS or Carbide cutters.
It also depends how rigid you machine is and good your spindle is.
Hard Maple and Birch are similar.
New sharp I/4" End Mill Cutter Carbide 1/4" deep.
Max Speed = 17121 and Feed = 4600mm/s = 15 fpm
This will use about .85KW on the Motor or 1.16HP
This can vary on the type of cutter and spiral angle.
So your Feed should be OK.
Is you spindle water cooled?
Does it have 4 bearings?
Does it have at least 2.2KW or 3HP?
You can get this info from using GWizard Machinist's calculator.
Regards,
Mauri.

Mauri, I kind of ran out of $$ toward the end of my build so just to get running, I put a dewalt palm router in as the spindle. (maybe this is the entire problem), but it was a cheap way to get running, or so I thought... 

Your spindle speed is slower than I would have thought!  17K RPM??

HSS cutters so far BTW... 

Soxfan963,
Yes this is your problem and the cause of a lot of most of your problems including lack of HP, RF Noise and Spindle capability with its limited bearings.
Your speed and feed are way too high for HSS End Mill especially on Hard Maple or Hard Birch.
Max on Cutting with HSS 1/4" end mill cutter and 1/4" depth of cut on Hard Maple or Hard Birch Slot Cut.
Speed 11900 Feed 250mm/min = .82fpm
You are lucky you did not break your cutter.
However if you are only taking only 1/16 side cut at 1/4" deep then.
Speed 13200 Feed 5000mm/min = 16fpm Climb Milling.
So as you can see depending on the cut you will need to vary your speeds and feeds especially using the Spindle you are using and the HSS.
Also if you push you spindle too hard you will wear it out sooner.
Regards,
Mauri.