If you are checking the temp at the exhaust with the transition hose on the temp should register closer to 160 degrees before the thermostat cycles off. Check to see if there is any obstructions in the lint chute assembly ahead of the blower wheel. I would also look to see if perhaps a couple of fabric softener sheets or something else may be blocking the intake to the dryer lint chute (look inside back of drum area upper right side). Also check to see if anything else is preventing air to be pulled in from the heater box. I've had a unit where the light fixture came loose from the back of the bulkhead. Air must be leaking somewhere if you do not get upwards of 160 degrees exiting the system.... Thus causing the heater to stay on longer.