Check all the seals in the unit.
The house air is being sucked through the drum etc. so is under a vacuum. If there is an air leak it will suck in room air and so the thermostat on the blower sees a cooler temperature and keeps the heater on too long.
I am starting to grasp at straws here.
Recheck that the heater is not grounded.
Unplug the unit and both wires to the coil.
Check it with a meter, should be around 10 to 12 ohms.
Then check from each side of the coil to the case/frame, both should be infinite ohms (open). If not the coil may have sagged or broken and is touching the case.
I doubt this is the problem but is another straw.
Did this dryer ever work OK for you.
Every once in a while I go in circles, only to find out that the dryer is new to the poster and the old owner messed with the wiring and that is the cause of the problem. Just want to be sure!!
Unplug the unit and disconnect it from the vent system.
Now try a run.
If it now dries OK odds are the vent system needs checking/cleaning.
Be sure to check that the louvers on the outside of the house open correctly.
If you do not want any lint in the house.
Take a pair of pantyhose.
Put one leg into the other and then attach this to the dryer's vent.
Leave enough room so the pantyhose can balloon out like a windsock.
Both the above will let you check the temperature and the air flow.
You could check the exhaust temperature.
On high heat the thermostat opens at 155 degrees F and closes at 130 so your exhaust temperature should cycle at approximately these temperatures.
Check that the lint filter and the holes where the heat /air enters the drum are clean.Sometimes fabric softener residue can coat these and cause problems.