Before doing anything drive the car to the tranny shop and have the computer scanned with a trans capable scanner. You may find that the problem is major but it could be minor too. Rather than pull the engine, at least at this point, the scan may tell you what needs to be done and it may be repairable in the car without pulling the engine and transmission. This sounds like the pressure control solenoid is going bad. That is why it shifts eratically. A scan will tell you what is actually happening before a major teardown.