I'm no expert on automatic transmissions, but it sounds to me like a problem with the vaccum modulator or pipe. If you can find a technical manual for your vehicle, I'd suggest locating the pipe and checking for leaks. The vaccum modulator measures engine load and manipulates the throttle valve, and if the vaccum pipe is leaking, you're likely to get sudden shifts in power output as the vacuum builds up and then bleeds away.
Again, I am no expert, and I'd recommend getting a second opinion, but that's my suggestion.
In related news, I hate automatic transmissions.
edit- I just got off the phone with my brother, who is more knowledgable about these things than I, and he says there could be a computer problem as well. What it might be or how one would go about fixing it, I have no idea.