Toyota Repair: 97 Rav 4 transmission issue, automatic transmissions, 3000rpm

Question
My Automatic transmission after 20 or 30 min driving, doesn't switch gears unless I hold the revs at 3000rpm. Then after 5 seconds it switches, and the tranmission light comes on. (as long as the engine light is on, it swtiches fine). If i stop driving for 30 min or so, i am good for another 20 or 30 min. I have changed the trans filter. What could be the problem?

Answer
Make sure the fluid is the correct fluid. Make sure you check the AT fluid when the engine is at running temperature. Lastly, I would have it diagnosed by a reputable shoppe. Automatic transmissions generally work great when it is serviced at regular intervals. By the sounds of it, it sounds like a switch or something actuated by temperature, it needs to be diagnosed to correct the exact problem.