84 300Dt- trans wont upshift, sometimes...

[RobTheMod]

Patient is a 1984 300D turbo, most of the time it shifts as nice as my well-adjusted, K1 kit equipped, just-serviced 300SD. But after a few miles, it likes to hang up in low gear when taking off. Sometimes even has to be driven home veerrryy slowly, because the upshift never comes. It hasn't been far from home because of this. I can't imagine it being internal, because of how well it works when it's working. No flares, noise, vibes, rough shifting, just smooth operation. Then she gets stuck in 1st and refuses to come out. Kickdown switch? Governor? Bueller? Bueller? Bueller?