It sounds to me that you have a sensor going bad they usally start getting lazy and are slow to respond. As I said earlier our sensor will climb to about 1100 degrees as mark said. They do however look lazy on a graph compared to your RPM. Typically on the track I will see a range of mid 800’s to mid 1100’s. I look at the longest straight in the track and by seeing that the temperature climbing the whole time I know the engine is burning fuel and is not lean, if the temp. starts to cool before the end of the longest straight (before lifting out of the throttle) I know it stopped burning fuel and went lean. The secret to max power and engine longevity is to be on that edge of lean and still burning fuel (climbing temp) without running out of fuel (cooling temp) running lean.
Another possibility is that your so rich that the engine is staying cool due to not burning all the fuel cooling the combustion chamber and that will keep the cylinder cooler because the unburned fuel absorbs the heat from the cylinder/piston and exits through the exhaust.
Rich=Cold, Lean =Hot, Lean is also faster up to a point, Lean is also dangerous to the engine causing seizures.
Most 2 stroke guys will tell you the engine never ran faster then what it did right before it seized.