Originally Posted by
Al1943
What kind of crankset is it? On a Shimano triple the big ring tooth count stamping should face away from the frame, the other two rings should face toward the frame.
Other brands are not that way.
That sounds like a potential cause to me. Chainrings wear where the front side of the tooth goes over the top. If you have reversed the chainring, you are mateing a new tooth profile to a worn chain. That would definitely cause it to skip in high torque applications.
If you really replaced the chainring only 400 miles ago, that's not very many. If, however, you didn't replace the chain at the same time, 400 miles is plenty of time for a worn chain to transfer it's wear pattern into a new chainring.