700r4 will not upshift at more than 1/2 throttle

My transmission went out on my 86 gmc sierra 4x4 about a year ago. Instead of spending the extra money for a BowTieOverdrives trans. like I put in my camaro, I bought a rebuilt trans. from Advance Auto. Well after 12,000 miles it has started giving a problem. Anything over about 1/3 to 1/2 throttle it will not not upshift out of 2 or 3. I took the pressures and they were a liitle lower than normal. I bought a Transgo shift kit hoping that it was just a stuck TV. valve. Well the pressures are normal now but it still will not upshift until I let off the throttle. It "surges" if I keep the throttle steady. I know that the TV is adjusted correctly and the lock up and everything else is perfect.

Besides needing a rebuild, does anyone have any other ideas? I hate to think that it is trash after 12,000 miles.

Thanks