Diagnosis
Yeah, this one's common. Unstable temperatures are usually a regulator issue. The propane tank's regulator has a safety feature that trips if it senses a sudden gas flow (like opening a valve too fast), causing it to severely restrict flow, which leads to weak, fluctuating flames.
How to Fix It
Do a full regulator reset. Turn all the grill control knobs to OFF. Close the valve on your propane tank. Disconnect the tank hose from the grill. Wait about 60 seconds. Reconnect the hose to the grill. Now, slowly open the propane tank valve all the way. Wait another 30 seconds, then light the grill, starting with the first burner and turning the knob slowly.
If That Didn't Work
If the problem persists, check for a kink or severe bend in the flexible propane hose between the tank and the grill. A pinched hose will restrict flow. Also, ensure your propane tank isn't nearly empty, as low fuel can cause pressure drops.
Heads Up
Never, ever open the propane tank valve with a grill control knob already in the ON position. Always follow the sequence: tank valve on first, then light the grill. Doing it backwards is a prime way to trigger the regulator safety lockout.