300D Slips Starting Out

My 1983 300D has developed a slip when starting out, similar to a slipping clutch in a manual transmission. It acts as if a band or clutch is about shot. The slipping appears once the car warms up (doesn't slip cold), and only appears to affect the start-out. As I understand it, the tranny was replaced with a rebuilt one about 30,000 miles ago, and seems rock solid otherwise (sometimes TOO rock solid). I replaced the fluid & filter about 3000 miles ago (the fluid looked pretty good), and the slipping has gradually appeared and worsened since then. The local independent MB shops seem stumped, and talk of either adjusting the modulator, or replacing the tranny (aahhhh!) Any ideas? Thanks!