Mine has this same problem moreso lately, I have had the whole system flushed about 8months ago. Changed the T-stat about 4-5 times (cause it's cheapest, and gasket), I'm leaning towards either the temp sender near the t-stat or the the other one (I was told there is another temp sender). Not really sure what the possibilites are and if it's the water pump I'd like to replace everything at once (T-belt, pump, sender(s)).
Anyone local to Taylor, Mi that would like some extra cash for some help....