Sometime won't go into Overdrive

To be honest it's 19 years old and if you start putting money into it you'll probably be sorry . Take a look at the frame and see how rusted it is then make a decision on keeping it

Toyota has a lockout system for the overdrive, the computer looks at all the sensor readings before allowing the transmission to shift into overdrive, I would have a live data scan done just to make sure the sensors are not sending any misreads to the computer. If those check out then it most likely time for a transmission overhaul as the pressure seals are leaking internally.