2001 hyundai santq fe

You don't specify automatic or standard so I'm going to assume an automatic. More information helps to assure you a better answer in the future. This should be helpful information for those who may encounter similar symptoms. A few years back I was building and owning transmission shops and now for hobby I build performance transmission and have designed and built my own for special projects. Before doing anything you should have it electronically diagnosed if the problem persists. If your engine light is not on then it may be providing what we call a soft code and goes away after you've shut off the vehicle and restart. Typically as the fluid gets hot (as you mentioned this happening in the summer) the fluid looses necessary viscosity and pressure drops. Your PWM will not deliver consistent pressure throughout the valve body and thus throughout the rest of the system. This drops pressure to the apply piston for the clutch pack which creates the "slip" or "flare" that shows an rpm jump. This usually will produce a soft code and will be found made identifiable with a hand scanner. Some models (not sure with the Santa Fe) have a simple key code method as in some Chrysler's ie; turning the key to on position three times consecutively will cause engine light to blink in flashing sequences thus "flashing" you a code number. Which you would look up to see what possible problem it would relate to. However a transmission shop or dealer will have a hand scanner to pull any soft codes or hard codes. All hard codes will cause your dash engine light to come on and typically stay on. The hand scanner will also identify anything related to the engine or transmission if there is a signal problem in communication from the ecm to transmission controllers, shift solenoids etc.

The typical problem is the transmission fluid getting to hot thus loosing viscosity. Most modern day vehicles have an adequate transmission cooler typically made part of the radiator. However if you live in a climate like Phoenix for example, an exterior trans-cooler is typically recommended. Specially start and stop traffic with road surface temperature well above 150 and cars being so close to the ground or air movement limitations. When the fluid startes reaching temperatures of over 250 degrees, transmission failures start. Anything over 280 to 300 will definitely create a temporary or permanent failure. There are more details technically to this but the above should suffice for your concern.