Trans slipping...need expert advice

[camry_y2k]

Hi,

I just started to notice what seems like trans slipping. It's intermittent at this point, but seems to happen from a dead stop, I felt the car not accelerating like it usually does, looked down at the gauges, noticed the rpm's slowly increasing where the speed was barely moving up, felt like something was holding it back, then then it shifted into 2nd it engaged really harsh. I might have noticed it also in 2nd gear but not as much. Overall trans feels sluggish, shift points seem high. Also when shifting from (N)eutral to (D)rive there is an instant jerk / harsh engagement, also happens intermittently. The car is a 02 Solara V6 (A541E) with 107k miles on it, trans has been serviced every 15-25k miles, fluid level and condition is good. Does not smell burnt. I've heard that common things to go bad with these and cause these kinds of symptoms are external sensors... VSS, TPS, even throttle cable out of adjustment etc.
Ironically I just replaced both valve cover gaskets, serviced the S/C etc about a week before I first noticed the slipping.
Please let me know if you have any suggestions and how to go about them, I would ultimately like to eliminate all the external things before I condemn the trans it self.
P.S. I have had the P0500 code pop up occasionally for the last 2 years and loss of speedo when that happens, but it always goes back to normal within minutes of it happening, but this hasn't happened for a few months now.

Thank you very much for any input.

Davor