Vacuum Modulator Valve. It's a $20 at any parts house.
Exactly the same symptoms, plus 2 qts. trans. fliud 'disappearing" into the intake each week, slow acceleration, and pinging.
I replaced mine last night. Two hours. Included a half hour attempting working from underneath before giving up since I had no picture of what I was looking for, then a 1/2 hour removing the console and pass. seat, and folding the carpet back & removing the hump access panel. Another 1/2 hour tinsnipping more room on the access hole and removing the trans. heat shield nut. Finally, the modulator is only held by a small stud and a vacuum hose, which drained a 1/4 cup of fluid!
I had done a search on this forum prior and got some good hints on the replacement. Thanks.