I dont know how good you are with setting gear mesh, but if you have it to tight it can be bad also. I put a piece of paper (printer paper) between the clutch teeth and the spurr teath, then puch the engine tight and tighten up the bolts. then pull the paper out. perfect gear mesh every time.

also are the bolts that hold the gear box in place loose? if they are then it may be twisting under power causing the gear mesh to change.