i have 1991 toyota p/u automatic transmission,when on freeway goes in and out of on 1991 Toyota Pickup

overdrive.

3 answers
DID THIS JUST START? WITH THIS ENGINE AND TRANS. SET UP IT WILL DO THAT,BUT NOT IN & OUT EVERY SECOND WITH STEADY THROTTLE.
IS ENG.RUNNING OK? IF SHIFT POINTS IN OTHER GEARS AT HIGHER RPM POSS. DETENT CABLE OR ADJUSTMENT.
that means the transmission is hunting for the right gear...due to possible sticking valves or low fluid.
Don't go on freeway.... BOTH are correct. Make sure engine is running properly and check fluid level (hot and on LEVEL ground in park. Use toyota fluid for best results or a good grade dexron III. If it's been more than 30K , it probably wouldn't hurt to change the filter (or remove and clean screen - I don't remember which they had). Also one wild thought.... if I remember correctly , that has a button to turn overdrive 'off' , is that working properly?