The a/t light IS the oil temp warning light. The a/t light fires up when the oil temp goes over some astronomical number like 273*F, and flashes for codes on the electronic shifted a/t's. Lay your hands on a gauge that reads high enough and test the a/t pressures. There's two taps on the dside of the tranny toward the back for line pressure and regulated pressure. I've never been into the ebox, but I bet the problem is with one of the duty cycle solenoids that regulate the pressures in the hydraulic circuits under tcu control. They tend to leak with age. Pretty cheap repair and can be done in the truck by dropping the pan. The hydraulic circuits are relatively failure free on this tranny, as they are almost identical to the pure hydraulic model, a very durable unit.

Aamco is a bad choice for transmission work, but they give a free diagnosis, and see enough trannies with problems to give you a fair probability of a good diagnosis.

I've found it to be a bad idea to flush an a/t. Usually causes more problems than it fixes.