three questions come to my mind:
1) is your saddle tilted too much, so that it's causing you to slide forward? It doesn't actually take that much tilt to make this happen.

2) is your saddle set too far back, so that your natural position from the perspective of overall fit is kind of on the nose of the saddle, so you ride there often, but then you scoot back to find a more supportive part of the saddle?

3) is the reach too long? this could cause both the scooting and the hand numbness.

It's probably not a bad idea to stop by your bike shop when they're slow and see if someone can observe you on the trainer for a few minutes. It's much easier for someone else to see exactly what's going on sometimes, and they may be able to tweak it in very little time.