Did you try the lightly touching the brake while holding constant throttle when your symptoms occur? If it goes away when you press the brake pedal just enough to make the brake lights go on, then your TCC is slipping.
Try new ATF first. If ATF is worn, this will solve it. Use ONLY MERCON V Automatic Transmission Fluid (ATF). Never Use MERCON/DEXRON III or anything that says "multi-vehicle." Castrol, Valvoline, and of course Motorcraft MERCON V work well and are cheap enough.
I dropped the pan myself and removed the valve body to get as much fluid out as I could. I could have drained the torque converter too, but my fluid wasn't THAT old. After cleaning the valve body and letting the transmission drip out over night, it took about 9 quarts to fill it back up.
If the ATF refresh does not solve the problem, you may need a new torque converter.