w124 transmission woes, doesnt seem to be obvious.

[mono]

first off, this is my first time posting on this forum, but i have been reading it for a long time. the advice and tech tips have been really invaluable. thank you.

now on to the fun:

1989 300TE wagon, 150K on the clock.

when the trans is cold, it shifts fine. shifting from R to D takes a second or two.

when the trans is at operating temperature, or we have been driving a lot in stop and go. the trans will not shift up to its top gear on the freeway. it is stuck in 3rd and only if the engine is revved much higher than normal RPM will it shift up. sometimes it would never shift up at all, and we would have to drive on the freeway at slower speeds not to reline the engine. now the situation has gotten worse, and it will now not shift from 1 to 2. if the car is put into neutral and revved, it will then upshift from 1-2. reverse works as normal when this symptom manifests. when refusing to upshift, the tranny also occasionally skips shifts and goes from low to top gear.

i've researched the b2 piston and k1 spring threads and i dont think this sounds like the same problem. fluid level and color are normal.

anyone have any insights as to next things to try? our shop is defaulting to "replacement transmission" without undertaking further troubleshooting.

TIA