91 Integra Sudden Overheating

This afternoon I happened to look at my temp gauge and see it approaching the red area. There was no noticable change in performance only the temp gauge nearing red line.
I immediately cranked up my heater and the needle dropped to around 3/4 range (1/2 being normal operating temp).
The the heater cranked up full, I noticed that at a stop sign, the higher than normal reading would drop nearly to normal. Highway, it is a little above 3/4 and going up a hill, the reading goes almost to red.
Also, at a cold start, the temp goes to normal, stays there for a few minutes then starts to climb quickly.
My coolant level is good both in the radiator as well as the overflow. I was thinking that the thermostat was sticking closed, but if it was sticking closed, hot coolant would not flow into the heater core and the heater would not be blasting hot air like it is.
Any ideas?
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machine snipped-for-privacy@yahoo.com wrote:

not after. using it therefore provides the /only/ cooling if the thermostat is not opening.
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Yes, most likely the thermostat. Replace with OEM. It's cheap enough and should be replaced every few years anyway.
Also make certain there's no rust, scale, or silt built up in the thermostat housing. This will interfere with thermostat operation.
And NEVER use silicated antifreeze, or tap water!!!! Non-silicated long- life antifreeze and distilled water are required. Use OEM antifreeze for best results.
While you're in there replacing the thermstat, make certain the rad is clear inside, with no deposits around the tubes, and that the fins are not corroded off the exterior of the rad. Run your hand across the rad in front, in the middle, at the bottom. If the fins are powdery and crumbly, replace the rad ASAP!
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How about your cooling fans are they coming on - ?

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