feedrate problem on circles

I normally cut fairly slowly, but I have been working on some plastic with small bits that need fast feedrates to keep from melting. I have a K2 KT2514 CNC with microsteping at 10 microsteps per 1.8 degree step with RhinoCam and Mach 2 and a 2.8 GHz desktop. I noticed that I could only get up to 20 IPM on the feedrate so I checked the post processor file and found a line in there limiting the feedrate to 20 IPM. I changed it and tried cutting around 40, but found on the 1/8" circles that I was cutting that it would sometimes make straight slots at the top of my circle. Seems to not like the fast feed rate for some reason.

Checking with K2 and based on the rapid manual travel I should be able to easily get at least 100 IPM and looking through the Mach book the software should easily make cuts that fast. Based on the limit in the post there is obviously some known reason that the speed is limited. Can anyone tell me what the problem is and if there is a way to get the speed faster?

I do have Mach 3, but have not tried the faster speeds on it yet. I am fairly used to 2 and they use the same post processor so I am thinking they will both have the same issue.