Diagnosis
This is the built-in overheat protection (thermal cutoff) doing its job. It means something is causing the heater to get too hot internally, usually blocked airflow or a faulty sensor.
How to Fix It
The number one cause is blocked vents. You need at least 3 feet of clearance in front of the output grille and nothing blocking the large intake grille at the back bottom. Move it away from walls, curtains, and furniture. Also, make sure you're not using it on a deep-pile rug, as that can block the bottom intake. Place it on a hard, flat surface. Clean the vents as described in the cleaning question. After it shuts off, unplug it and let it cool down for 30 minutes before trying again.
If That Didn't Work
If you've confirmed perfect airflow and a clean unit, the internal temperature sensor might be faulty and too sensitive. Try running it on a lower heat setting. If it still cycles off prematurely on a low setting in an open area, the component regulating temperature is likely defective and the unit needs service.
Heads Up
This automatic shut-off is a critical safety feature. If your heater stops doing this and just gets dangerously hot, that's when you have a real problem. The fact it's shutting off means that safety system is, at least partially, functional. Never disable or bypass it.