There'* a Sonnax kit available that replaces the chronically failing TCC solenoid bore and will fix the problem you are having. I had this done to my current transmission. It consists of a new bore sleeve for the torque converter clutch apply solenoid in the valve body, a new solenoid with a larger piston, and it works freakin' awesome.
What happens to cause this is the bore becomes oversized due to wear from the TCC solenoid piston and allows too much slippage to occur, which the PCM then sees and pulls it out of overdrive and into it'* fail-safe mode, 3rd gear. There is a way to force the car to run in overdrive without having complete lockup, but I suggest not using it because it will fry your transmission.
This repair can be done with the transmission in the car, but it is one that I will NEVER suggest that anybody without some transmission experience (beyond a filter and fluid change) attempt to tackle. It involves pulling the side cover and the valve body, drilling the bore out to a pre-determined size, installing the new bore sleeve, and reinstalling the valve body and all related solenoids.
Keep in mind that even if the transmission was rebuilt recently, that doesn't mean that they didn't reuse the valve body because no wear was detected at that time. It'* very possible (and very likely) that the bore was already showing signs of wear but not enough to cause any concern at the time of rebuild.
'94 Bonneville SLE | 266K miles
CAI - K&N cone filter - polished intake runners - 140 amp alternator - 2.5" exhaust - Magnaflow hi-flow cat - Magnaflow dual outlet muffler - 106K on engine and transmission - Sold 1/14/07
94 VW Jetta - 151K - 2.0 - 5 speed - Ghetto black steelie wheel mod - Techtonics 2.25" cat-back system