300SEL Shift Help (2nd Post)

[90mbenz]

My 1990 300SEL (190k miles) seems to be developing tranny shift problems. Fluid and filter have about 15k miles - no leaks, odor, or burnt color.

The problem does not happen all the time and seems most likely after long drive - 30 minutes or more. If cruising down the highway and the accelerator is lifted, tranny downshifts into next gear and won't go back up unless car is slowed or stopped. Then it upshifts as usual. Wife just reported that while backing out of the driveway after a 2 hour trip, the tack raced a bit and it 'didn't feel right'.

So, looks like we've either got slippage or shifting problem. What to look for first? I'm thinking vacuum for the modulator? Any help appreciated.