Try a pan drop, filter change, trans-go shift kit and refill. Could throw a bottle of Lucas in it if you want. Might stave off the inevitable. My 00 GT does this when it gets hot out sometimes, wether there are other people in the car or not. Usually when I get stuck in stop/go traffic and it heats up.

Have you checked the fluid level in the trans? Remember- fully hot, at an idle ni park and on a level surface.