Try easy things first. As suggested by JohnDThompson try a pedal swap. Also try thoroughly cleaning both pedal and crank threads and use grease on the threads and mating surfaces, and perhaps install pedal washers, when reassembling; torque correctly. Try greasing the mating surfaces of your axles, quick releases and dropouts and ensure that the QRs are really tight. Chainring bolts are another potential trouble spot; I clean them and their mating surfaces and and grease only the male threads.