2004 H2 Engine Temp Gauge goes Nuts

[Olfart]

2004 H2 49,000 miles
Temp gauge normally reads OK...abt 200 degrees. But recently once in a while it slowly climbs until it reaches max temp and when I cut off the engine it reads about half scale instead of going back all the way to the left. It continues this for a day or so and then finally returns to the correct reading. The engine is not overheating. The only thing done to it which might be related is the replacement of the water pump about 3 months ago.
Purchased the Hummer used with 39,000 miles about 3 years ago and love it although I have had alot of work done to it....thank God I purchased a 3yr extended warranty...althought that has now expired.
Tnx for any suggestions on the temp gauge problem