Diagnosis
It's hitting its internal overheat protection (the thermostat or a high-limit switch). This happens if the air intake or exhaust is blocked, or if the unit is in too small of a space.
How to Fix It
Check the basics. Is it at least 3 feet from any walls, furniture, or curtains? Is the rear air intake completely clear? Vacuum the intake grille to remove dust bunnies. Also, make sure it's on a hard floor—using it on plush carpet can restrict the bottom intake and cause overheating.
If That Didn't Work
The internal temperature sensor might be faulty or degrading, causing it to trip at too low a temperature. Or the fan motor could be slowing down, not moving enough air to cool the element. You'd need to open it to check the fan for obstructions and test the sensor.
Heads Up
This shut-off is a critical safety feature. Don't try to bypass it. If it's tripping even with perfect clearance and clean vents, the unit has an internal fault and should be retired.