So to start, if the thermostats by the element are tripping, it means that the air isn't flowing properly because those thermostats only cut out the element if the heat is rising and not circulating.
You'll need to remove the the front panel of the dryer and clean the vent below the lint filter. Then spin the blower wheel attached to the motor and see if the blower wheel is stripped out and not attached firmly to the motor shaft anymore. If it's stripped out then the blower wheel needs to be replaced.
Other than that, you really need to determine which thermostat is opening up and cutting power to the element. Probably the easiest way to do that is to tape the 2 wires going to a particular thermostat together with electrical tape to just bypass that thermostat. Then run the dryer and see if the element kicks off after 5 seconds. If not, then that is the thermostat you need to replace.
Also 2 things I want to add. Make sure the vent from/through the house is not clogged or that can stop the air flow as well. And the last thing to add would be that the front panel of the dryer as well as the drum and everything other than the top panel must be installed when testing or the air flow will not flow properly.
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