Wacky Temperature Gauge!

I have a 2001 ZX2 with about 120,000 miles on it.

Recently, the temp. gauge on the dash has become possessed. It always starts at "cold", but then will quickly shoot up to the normal range, then go back to cold, then shoot up near "overheat" then drop back down and so on.

While this is happening, the heat coming out of the vents follows the gauge on the dash. That is to say, if the dash reads cold, the air is cold. If the dash reads hot, the air is hot. So the air coming out of my vents keeps switching from cold to hot and back to cold.

I recently had a problem with the gauge not working at all, and replaced both the temp sensor (inside the housing with the black hose), and the sensor that goes into the top side of the engine block. This fixed the problem for about 8 months, but now I am once again driving around with one eye on the gauge, hoping my car doesn't overheat.

Any help would be appreciated! Coolant levels are fine, and there are no leaks of any type on the car. Thanks!

Uh, it seems too obvious, but have you replaced the thermostat? Perhaps it's sticking.
I recently replaced mine and when the old one came out it didn't appear all that worn. There was just enough play/wear on the centre shaft to allow the valve to become lodged askew and would not open completely. I popped a new thermostat in and haven't had any problem since.
Thankfully, I was able to get the car off of a multi-lane highway when I noticed the gauge creeping up to the max before it started to cook! No damage. :)

Thanks for the feedback. I replaced the thermostat about 8 months ago, but if it seems that obvious then maybe I got a defective one or just didn't put it in right. I'll check that out this weekend and update this once I try.

Thanks!

Replaced the thermostat and it seems to be working now. Keeping my fingers crossed I'm not back in another 8 months asking the same question!

Thanks for the feedback!