This exact same thing was happening to me about a year ago.

I had a drive train overhaul done. Didn't fix it.
I installed a new chain. Didn't fix it.
I replaced the rear cassette. Didn't fix it.
Finally I replaced the chainring I used most. No more slippage.

The only time I noticed the slippage was when I was really pedalling hard. When I compared the new chainring with the old one, there was definitely wear, but it was not as pronounced as I would have expected, but the concave section between the teeth was definitely asymetric, with wear on the trailing edge that pulled the chain around. For me this occurred with about 8,000 miles on the chainring.

Trekaholic