Diagnosis
This is likely the built-in thermostat doing its job, but it might be reading the room temperature incorrectly due to its placement or a draft. The DR-968 has an adjustable thermostat dial that cycles the heater on and off to maintain a set *ambient* temperature, not to run continuously.
How to Fix It
1) Check the thermostat dial on the side. If it's set low (like between Min and 65°), the heater will shut off once the air near its sensor feels that warm. Turn the dial to a higher setting (towards Max) and see if it runs longer. 2) Relocate the heater. Its temperature sensor is inside the unit. If it's in a corner, behind furniture, or in a direct draft from a cold window, it will get a false reading. Move it to an open area where it can sense the room's average temperature.
If That Didn't Work
The auto-shutoff timer might be activated. Some models have a 12-hour safety timer. If you've had it running for a long stretch, it may shut off and need to be manually turned back on. Unplug it for a minute, then plug it back in and restart.
Heads Up
Don't confuse the thermostat dial (which controls temperature) with the heat setting switch (High/Low/Eco). The Eco mode uses the thermostat aggressively to save power, so it will cycle off more frequently. For steady heat in a very cold room, use High or Low heat setting and set the thermostat dial near Max.