Aaah! (Magically Moving Steppers)

Aaah! I spent some time trying to fix a problem with my parts coming
out lobsided, and just believe I may have found it.

My system is on, then I turn on the Spindle of the Sherline, turn it
up to max. The Sherline spindle is not connected to the CNC system,
it's the plain old power switch on the spindle of the mill.. I turn
the speed control up to max.

I then watch the rear-shafts of the motors, and though they were
holding still before, they now roll back and forth every few seconds
by a small amount. Aaah! I'm thinking this is due to induced mains
hum throwing the dir/step pins. Any advice?

My system is a little limited still, so it's basically:

Xylotex driver board
Switched Mode PC power supply (12V driving the steppers)
BF Cap 47,000uf
Some big NEMA23 steppers

Could it be the switched mode power supply in some way? It driving
the logic also, I understand that switched mode PSU's employ PWM to
provide the correct power level (just like a chopper board), could
this be interefering with the system?

Any advice greatly appreciated.