Hello,
Check the engine timing. Engine timing will effect engine vacuum which will alter transmission modulator pressure.
Check for vacuum leaks.
Check the function of the three position solenoid and its linkages
Next I would remove the transmission pan, check for any broken parts or debris. If none, I would remove the filter and re-toque the valve body in the transmission (10mm head bolts). There is a small chance that the bolts have gotten loose over the years and are causing an internal fluid leak. Change the fluid and filter while your at it. Use the recommended fluid and drain and change the torque converter also.
Consider going through the modulator adjustment procedure after all else fails. (Somewhat difficult)
Check back for more specifics if needed (lots already posted on these issues)