transmission problems

When you put it in gear you step on the brake, right? And it dies when you step on the brake?

Would be worth checking your power brake booster and booster check valve. Disconnect the hose at the engine, plug it, and see how it drives. If the booster or check valve is bad its a big vacuum leak.

Your problem sounds like it is due to the torque converter. Sounds like it is locked up. I'm not familiar with that transmission, but it may have a solenoid that locks up the torque converter when cruising, which gives better gas mileage. Does your problem only happen after the engine and trans are warmed up? IE, after 5-10 miles of driving? If so, then this solenoid is the problem. The solenoid is sticking when it gets hot. Find the solenoid and replace it. Or... just unplug it. If you unplug it, it won't go into lockup mode. It's fine to drive it this way, you just won't get the small fuel economy benefit.