I have the whirlpool Cabrio dryer model WGD6400SW1 that heats up for about three min. then blows cool. I’ll set the time at 40 min it warm to about 36 min. then cools off the time will jumps from 36 min to 16 min. then to five min. before counting down and turning off. I replaced the exhaust thermistor and tested it with a multimeter tested fine still showing with diagnostic test F22 and F24. What can be the problem?

Seeing as there is not a designated F24 error on that model and you changed the thermistor and still get the F22 error and I assume you have verified the wiring is ok you need to replace the main control on the dryer because it's bad. The board you need for that model can be ordered from the link below:

https://prismparts.com/products/whirlpool-dryer-main-electronic-control-board-pp4805106?_pos=1&_sid=ebce6b47d&_ss=r