I have a 2001 Elantra with an auto transmision. Car only has 52000 miles. When I drive at high speeds for extended periods of time, 75 mph for 45 minutes the car appears to randomly shift into 3rd gear and my rpms shoot up to about 4500. When i pull over to stop and start again i have almost NO power, almsost as if the car is stuck in 3rd gear. I can manually make it go into first and get going again. When this happens the check engine light comes on. When I turn the car off, and start it again, the check engine light is off and I can drive it another 45 minutes at 75 mph before it happens again. Is this a transmisions slipping? Doesn't seem to be as I'm not in the process of switching gears and I'm just cruising along. Someone told me it might be a computer problem. Any suggestions?

looks like the transmission is slipping, you need to check the oil level, also a oil flush could be good.

Do you have a check engine light on? It sounds like the car is shifting into what they call a "safe" mode...ie. third gear. The lack of power when taking off from a dead start or slow speed indicates it is probably in 3rd gear, which is what it is designed to do when it "senses" a problem. Try taking it to one of your local auto parts stores and getting them to put a trouble code scanner on it to see if you have any trouble codes showing up. Could be the input speed sensor and/or output speed sensor. I have a 2001 with about the same mileage and just started having trouble. Mine showed the speed input sensor being bad and I had it replaced. Part is about $80 and relatively easy to replace. Dealer will rape you. It did make a difference, but I am not convinced it is shifting in the right sync all of the time. My gas mileage is cut in half. If you have never had the transmission flushed and new fluid, then it couldn't hurt. I don't think the maintenance log calls for it that soon though. If you do anything with the transmission fluid, make sure it is the approved Hyundai fluid. I would suggest not buying it from anyone other than a dealer. It is expensive.