RPM drops to almost 0 then goes back up

RPM drops to almost 0 then goes back up, this happens when i shift from r to p or p to r

It sounds like a crank position sensor going bad, but it could also be a bad motor mount allowing the engine to move around too much and pulling on the crank sensor wires causing the rpm drop.

I think the Crank Position sensor is the best suggestion, but I had a bad engine to H Frame ground that caused this kind of problem. I ran a new 18 gage wire from a bolt on the engine to the h frame and it solved all problems.