motor inteference?/loosing steps

Hello, I'm new to the group and cnc thanks in advance for any help!

I recently bought a taig mill with sherline steppers, denvercnc
control and running Mach2 controller on winXP home 2400Mhz dell
computer.

I'm having problems with what I believe is interference from the
spindle motor or general A/C noise.

If I leave the mill spindle running and have Mach2 doing nothing
(holding the steppers at 0,0,0 and wait about 2 minutes and then
check my handwheel zeros the steppers will have moved .002 to .035
(or more). Mach2 still says 0,0,0. I can observe the
servos "glitching" randomly, but they always seem to move in one
direction. If I switch the mill motor on and off or switch my
Sherline lathe motor on and off nearby 90% of the time one or more
steppers will glitch.

I tried running power from a different circuit down to the motor
(extension cord) this seemed to help a little (or maybe wishfull
thinking?) but the problem is still there.

Yesterday I mounted a sherline motor/head/speed control on the mill
and the problem got worse. It's there with the Taig or Sherline
motor though.

Should I try grounding my mill? Should I ground mill, controller,
computer all to the same point?

Thanks for any suggestions,
Andy