I get the same feeling sometimes too. I feel this *pop* sensation in my feet sometimes when I'm starting from a standstill, or after really mashing up a hill. It almost feels like the chain is trying to jump off the ring.
I've been trying to track it down for the past week. So far:
0. cleaned and lubed chain
1. Replaced BB (old one was dead)
2. tightened chainring bolts
3. tightened cranks
4. checked pedal tightness.
5. checked chainline (about 1mm off)

This has reduced the popping by about 75%, but it's still there. I guess i'll tear it down again this weeked. and check the BB tightness.
Maybe I need a new chain?