Do I have to go to servos?

I finally got my mini mill tuned in and I was running a circular
pocket program; everything was going great until Z got to -.092 and
then the X and Y started to wander. I know both of them are wandering
since the gouge is in middle of the x- y+ quadrant tangent to the
circle, not at the top or bottom of the circle.
The walls of the pocket up to that point where perfectly straight and
I had been watching for wobble so I believe it hadn't been wandering.
The load wasn't great, the wiring is separated and shielded and I
have an EFI filter on the power supply. I am using gecko G210's on
all axis but they are really 201's since I pulled the multiplier
boards and Mach2 V6.06.
This process was running a long time (about 20 minutes) since I was
running a feed of 5"/min and a .004 plunge on the Z axis.
Do I have to go to servos to get a reliable movement over a long
period of time.
My last resort is to take the entire system over to a daedal setup I
have and run hours of tests to see if I can duplicate the problem.
This is the only way I know to eliminate the mill as the problem
source. If I can't duplicate the problem on the daedal, meaning that
servos will not help, then will I have to scrap the mini mill and
start over again (for the third time)?