After three pulse generators on my 1999 Tiburon it will go days even weeks and then it starts shifting into second gear at 40 mph then has no first or overdrive Could I have a computer problem?

Yes, it's possible that you could have a computer problem affecting your transmission. The symptoms you describe—shifting into second gear at high speeds and the absence of first gear or overdrive—may indicate that the transmission control module (TCM) or the engine control unit (ECU) is malfunctioning. Additionally, issues with wiring or sensors related to the transmission can also cause erratic shifting behavior. It would be advisable to have the vehicle's diagnostic codes read to pinpoint the exact issue.