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Unstable throttle (rapid up/down movement) while in overdrive, 1989 Isuzu trooper

The trooper has a 4 cyl. fuel inj. w/auto tranny. As of late, the tranny will shift intermittantly from overdrive down to 3rd then back up/ back down until I step on brake lightly then it stops shifting. This all happens when above 30 MPH and ONLY AFTER the first 15 minutes when started cold. The up/down shifting occurrs at low as well as high engine vacuum. When placing the tranny select in 3rd the violent up/down shifting stops but it runs at 3000 + RPM. The shifting while in overdrive is rather constant. I have checked and rechecked for vacuum line/hose leaks but found none. Can’t find anything on subject in repair manuals either. HELP!!!