After a little more thought and research, I think what is really happening is I'm slipping in 1st gear (ie sluggish takeoff from a full stop). I'm thinking of still dropping the pan and taking a look (plus new filter, gasket, clean gov pressure sensor and soleniod), but the repair manual suggests slipping may be caused by a myriad of things. Thoughts?

Also, to clean to the gov sensor and solenoid: do I just wipe off and reinstall, or clean gunk with a solvent?

Thanks!

 

Final comment/closure:

Replacing the Gov. Pressure sensor and solenoid cured the problem. The transmission was not slipping. Rather, it was not downshifting into first gear, so the sluggish takeoffs were due to starting out in 2nd gear.