Sounds like some thing is binding. When I put the metal gears in my Tiger never had to brake them in it ran fine from the start. Seeing as I do not have a Pershing I don't know about the gear box shape is it the same as the Tiger? they sell a gear box for the Tiger and the Pershing that would lead me to believe their is a difference. I would try running the tank with the top off to see if it is binding with the upper hull that and you could see what is going on in side.

Jeff