This is annoying and weird
 

Symptom: When cruising at around 55 mph, and about 1500 RPM's, under light load, as soon as I let off of the gas pedal, the RPM's drop IMMEDIATELY down to 1000, and then auto-correct themselves back up to around 1300 or so. It is accompanied by a small loss in engine power, which is restored once the RPM's go back up. Its really annoying. I know its not a tranny slip, becasue if it were slipping from 1 gear into a lower gear, the RPMs would go up, not down.

Theory: Bad TPS. The TPS is worn in that spot around 1500 RPMs, and so instantly falls down, cutting off some air from the engine, causing the loss of power. Once the TPS hits a spot it is not warn in, it corrects itself to a proper speed, increasing the air flow, and restoring the power. What do you guys think?

I'm thinging yout TC wasn't locked and when you let off the gas it locked. If you let it go even longer the RPMs would drop back down again.

The torque convertor locks up just fine, because my RPMs at 60mph are 2000 and at 90 mph are 3000. It has to be something else

but if the TC is locked and you let off the gas the RPM's won't change.