I got a high mileage 85 500 SEC. Car sits during the week
and I only drove it around the block just to keep the
battery up on weekends. I put in on the highway and noticed the temp gauge going up so I pulled over to check coolant level.
After the temp went down I put it into drive and now it will not
upshift out of low. Just low, neutral, reverse, and park gears work now.

A mechanic dropped the tranny pan and said he found a plastic
piece which he replaced "bushing". Although no metal
contamination was found he also replaced the filter and fluids.

I go to pick up the ride and the same problem is there.

Any ideas or further observations I should also pay attention to?

I just got the thang and 3 weeks later here I go with this mess.