Sluggish auto tranny--99' GCL

[Goldfishy]

I recently changed the tranny fluid and filter (OEM) with MOPAR 4 ATF, with 188,000 original miles on it. The magnet has very little fine metal dust which I considered par for the course. The tranny shifts fine. Tranny level is on the mark.

My question is that in the early morning after being idle for 10-12 hours overnight, and starting the car for the first time, and letting the engine/tranny warm up after 1-2 minutes, when I put it into gear, it's sluggish in moving, sometimes even like it's slipping to catch up to the RPM's.

It only happens in the first few minutes. After running about 4-5 miles, it's all good and once the engine has been running, it's great. Leaving work after 8-10 hours and knowing the engine/tranny is warm, it works fine also.

What could be the issue going on in the early morning?