The likely culprit is the transmission but there may be other problems also. This kind of problem is very difficult to diagnose on a Forum like this because we really need to be there to see and hear exactly what is going on. If you see oil splatters on the inside of the cabinet this is further evidence of a transmission problem.

I would start with replacing the tranmssion and see if that fixes the problem. First, think about whether or not you want to make the repair as opposed to replacing it since it is an expensive part.