1997 Grand Caravan 3.8L 4 speed, seems to slip out of gear and not move when starting out cold.

This issue seems to occurring more often now.  When it first starts up, it will go into reverse to get out of garage, but will seem to slip out of gear when you shift to drive.  It only does this in the first few minutes.  Afterwards it shifts smoothly and problem doesn't repeat itself for the rest of the day.  We changed out the pump filter and fluid, seemed to make it better for a few days but problem is back.  Km count on van is 328,000 but engine is great, not sure if this is a torque converter problem - it's probably the original part in there.