On some but not all laptops, configuring windows power saving to PC mode doesn't cure all the ills. Some of them are buried deep in the BIOS and you can't get rid of them at all.

Also - Windows isn't and never will be, a real time operating system. Anything which could interfere with the speed at which the stepper pulses are generated could be the source. And, unfortuately, something that works on one machine, might be death on the next machine.

I would be suspicious of AVG as well. Don't get me wrong, AVG is good and not a particular CPU hog, but anything other than Mach itself, should be treated with suspicion. Even a screen saver could get in the way.