There are a couple possibilities. The first would be engine performance. If the engine is lacking power during warm-up and requires more throttle then the other two factors are affected. Those factors are the control pressure cable and the vacuum modulator.

If the engine is right then I would make the trans shift earlier (lengthen the control pressure cable) and/or make it shift softer by backing off the vacuum modulator.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician