Take it to a transmission shop, get a diagnosis for free from them. They'll tell you what's going on. One thing I've learned about Trannies, if it's got something wrong, do NOT drive it anymore. You might be able to save it for a couple hundred bucks at a Transmission shop, whereas it will be 1,500 or 2,000 dollars once the tranny dies full blown. You might have to order a whole new rebuild, although you do have those cores. It's up to you really, but your son is going to be stuck on the way to work one day, and he's going to lose money to that tow-truck man.