well, the laptop was a fresh install. It has no other programs on it. It has a wireless network card, but I disable that before I load mach.
I have been struggling with this issue for a while and over the course of time I have stripped the poor thing of every possible thing I can.. msconfig is my friend and every "extra" startup item was removed.
The P4 1.7 desktop was not a fresh install, it too was stripped down.. The only real software on it is Rhino3D. LAN card was disabled.
None of this had made much a change.
This morning I got my hands on an AMD XP 3000+ machine with winXP Pro. It has many programs loaded on it. I have not attempted any optimization changes on it at all, or even disabled its network. I loaded mach3, set it to 45000hz, and the thing is very stable on the Pulse Frequency.. Only twice now I have seen it drop by exactly 3 hz. The "Dbl Plse" is at 3 and does not change at all.
I have not had the chance to run the mill on this computer yet, but it looks promising!

Art- Is the "Dbl Plse" a hint at all? Stable at 3 on one computer and way all over the place on another mean anything?
thanks!
colin