Have a different mechanic at a different shop look at it. or try to fix it yourself
Some things to check for
Chainring orientation on the spider. The shift ramps need to be in the correct position
Warped big chainring
Bent or missing chainring teeth
Loose chainring bolts
Crank side to side play/loose crank bolt. Incorrect bb spacers/dust covers installed
Twisted chain
Are you a heavy rider ? The crank and rings might be flexing under load.
Last edited by trailflow1; 11-15-19 at 08:13 PM.