I've combed the vacumn charts threads but haven't found a description of this exact issue.

When the car is cold, all is well. Shifts are smooth and not hard or clunky at all. Once it's been on the road for an hour or so, the transmission seems to want to downshift, or stay in lower gear. I can be going down the freeway at 65-70, and suddenly she'll be down in third. She won't upshift to 4th until I get the Rpms >3500, and then she'll pop into 4th...but if I slow down at all, back to third she goes.

Where should I start looking for the problem? Feels like the vacumn modulator if I were to guess, but I don't want to guess. Thoughts?

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Lynrd

The BBB - 1991 350SD, 166,000 miles
1993 HD FXRS-CONV
1988 BMW R100RS