Thomas,
I have Mach3 running on 32 Bit Windows 7 and it works faultlessly so it's almost certain to be something in your config or hardware that's causing your problem, I did find that copying across my xml config file didn't transfer all my settings for some reason but once manually entered, including some I'd forgotten about, it's all working perfectly
Have you checked your de-bounce timing is the same as it used to be?
Regards,
Nick