I just had a problem very similar to this. Unfortunately, I had to have the tranny replaced to the tune of $2200.
There may be other problems going on. A good way to test is check the tranny fluid. If it smells burned and/or is not a reddish-pink color then you have a problem. If not, there are sensors and valves that may not be working properly and need to be replaced.
Your best bet is probably to take it to the dealer or a transmission shop.
