Hold button and 4th gear Help Please.

[KariBri]

Hey I just bought a 1996 Kia Sportage and it is not shifting into 4th gear. I noticed it has something to do with the hold button. When I push the hold buttton before I shift into drive it will go into 4th gear but only for about 5 minutes and then it shifts back to 3rd while driving. I'm not sure if it is in the hold button or the shifter. But I dont think it is in the tranny. It runs good with no slipping. Any Ideas? Thank you for any help.

[LMP]

Is the check engine light ON ?
NOrmally, when youturn the key to ON and start the engine, the HOLD light is OFF and in tht consition, car should start in 1st gear up to 4th...but in fact 4th engages just once engine temperature is above 140°F
If for some reason the thermostat is stuck open and engine cools down below that temp on the open road, it will revert to 3rd......or if some false temp indication is fed to the computer.
Start with those ideas and come back with findings.