2003 Pontiac Bonneville Clunks hard into gear at low speeds

I sold a friend this car with 70k miles on it. Now, at 90k, it has developed a problem at low speeds only. When stopping or starting it comes out of gear and then slams back into gear quite roughly. He had it diagnosed @ 3 different transmission shops, all 3 told him it was the pressure control solenoid with an estimated repair cost of $300.00 to $1800.00. He elected to have a shop fix it for 400. repairing the solenoid only. When done, the shop told him it is better, but still does it and they suspect it may be the throttle positioning sensor. If so, I would suspect that the problem was the TPS all along, NOT the transmission. What is your opinion on this? The shop still has the car and it is still not fixed. What could be cusing this and how can 3 shops incorrectly diagnose a problem?