You said you had gone from platform to clipless pedals so this may not be the problem. I had a similiar issue. Like you did everything trying to find if. After reading some google search I had done I removed my cleats, cleaned and put antiseize on the bolts. Reinstalled them and applied a light coat of chain lube to the cleat and let it sit over night. It solved my problem. Periodically I apply a little chain lube again and let them sit over night. Haven't had the problem since.