Auto Xmission Problem

I have a '99 Ford Ranger 4.0L with automatic transmission. The past several weeks I noticed that during slow acceleration from a dead stop my tranny jerks when shifting from 1st to 2nd gear. Anything after that the tranny shifts smoothly. Also if I accelerate faster from dead stop, the jerking motion seemed to be less pronounced or unnoticeable at all. At times however (not too often) at low speeds it shifts down suddenly and back up again. This does not happen during high or moderate speeds. Just had the tranny drained and filter replaced and fluid level is OK. No evidence of any abnormal pressuization and/or overheating in the system. No leaks. Anyone got any idea what's happening?

modulator valve???