I had the same problem. It turned out to be a mesh problem. I ended up putting 2 shims on one side and 1 on the other. I also have the Integy cases and they are a bit of a pain to install. It took me 2 whole nights of troubleshooting to find a sweet spot. Have you tried any shim combinations yet?

The only problem I could actually diagnos was the center drive gear was "sliding" into the gear case just enough to put everything in a bind. At first, it was completely locked in place. Then, I began to mess with the shims trying out different combinations until the mesh just felt right. After several, and I mean SEVERAL attempts, I finally got it.