Diagnosis
Yeah, this one's common. The built-in thermostat is a simple ambient air sensor, not a precision device. It's also located near the top of the tower, so it reads the warmer air it just heated.
How to Fix It
Try setting it to the highest heat setting and desired temperature, then use a lower fan speed. This can sometimes smooth out the cycle. Also, make sure the heater isn't tucked in a corner or behind furniture—it needs to read the room's general air temperature.
If That Didn't Work
For more stable control, consider plugging the heater into a separate, plug-in thermostat outlet. You'd set the heater to high heat and let the external device manage on/off cycles based on a sensor you place elsewhere.
Heads Up
Rapid cycling (on/off every 30 seconds) is hard on the relay and motor. If it's extreme, it's likely a faulty control board. Using the 'ECO' or auto mode sometimes exacerbates this.