Odd temperature guage behavior

    We have a 95 Eclipse GST, stock.  Bo-ring, I know, but it's my wife's car.
    I recently noticed something odd with the coolant temperature reading.  For 13 years, the needle on the temp guage has always locked at mid-way after warming up, never varying. 
    Two days ago, it suddenly started rising a little, to the top of the thermometer icon on the guage, not to the upper danger limit.
    Radiator is clean, fluids at proper levels, system was recently flushed and replaced, fans are operating.
    After test driving and experimenting, I concluded that driving without the turbo, and at lower rpms, the temperature behaved.  Upon higher acceleration, invoking the turbo, the needle would quickly move up to the higher position.  Ok, I thought, the extra heat from turbo driving is causing the higher temp.
    The really odd thing is that with the needle at the higher level, driving on the highway, when I depressed the clutch and let off the gas, the engine going to idle, the needle quickly returned to the midpoint.   Releasing the clutch and depressing the gas again, the needly quickly climbed back higher.  I did this several times and it was consistent.
    The needle movement changing with RPM seems much faster than the water temperature could be actually dropping.  Has anyone seen such odd behavior from their water temperature sender or guage?   Or are there any other ideas?
    Thanks in advance.

Weird. I'd start with the thermostat. Throw it in a pot of boiling water, and if it opens then it's still good.

I bet it is the thermostat. It is probobly not opening all the way. It is an easy and pretty cheap fix if you do it yourself. I would try that first. It probobly isn't the sender because it would read wrong all the time. I would say that it has got to be the thermostart or water pump.

Thanks, I thought about the thermostat -- but it climbs only so high and stays there, and why would the temp immediately drop to normal when I let the engine go to idle, and vice versa?

I also read, after posting, about a glazed water pump belt which slipped at higher RPM but not at lower.  Thoughts?

Thanks for your responses.  Not going to rag me about my wife's stock GST?

y would we rag on a gst?lol we'd rag on you if it was an rs or gs,lol

Also take a peek at your fan relay under the hood. If it's no good then obviously it would cause your fan to not work and temp. to rise but it's more than likely the thermostat as stated.

Yeah, think about it this way, if the thermostat is stuck and it won't open all the way then at higher RPMs when your pumping more water you are going to have restriction, it makes perfect sence.

I would agree with everyone here man, go buy a thermostat. They are not hard to replace and shouldnt cost you no more than $15.00 with the gasket and sealant

OK, thermostat it shall be.  This weekend it'll warmer and maybe I'll bust my knuckles (and cuss) a little less. 

Thanks again for your comments.

It is REALLY easy to do just make sure that you drain your coolant first.

Well, guys, it apparently was the thermostat.  I replaced it, took Daemon out on the highway and hit it hard (for us) and the temp guage needle stayed right in the middle like old times.

I knew the thermostat could have been the problem, but sometimes I tend to overanalyze symptoms -- you pushed me into going ahead and doing it, so thanks again!

No Problem!

While your under the hood go ahead and check your hoses to see if they have swelled from it overheating. You should be ok, but just check as a precaution.