Temp gauge going funky, Updated! Changed Thermostat but still does the same prob Options

[reko]

As i was driving on the highway on friday night i noticed that my temp gauge was almost at cold which isnt normal for a sunny summer day.

After rolling for 10mins, it went to normal temp. but started to go slowly from normal to cold back and forth 4 to 5 times after what it stays at the first 1/4.

It did it only once, but then on the next day, it did it 2 times and went back to normal.

Nothing happened yesterday, but still, i dont really like this.

I'm thinking thermostat, but who knows?

Is it bad? I mean I went to school today (which is about 20kms from my house)... should I change that tonight?

**UPDATE**
25/08/08: Changed changed the thermostat... The shaft bent so it was stuck open so I just put one for Camry since the one for Celica 2.2L didnt fit (dont know why but anyway....) Still does the same problem as before... temp gauge isnt stable...

So what's next?
Water pump maybe

This post has been edited by reko: Aug 25, 2008 - 5:47 PM

[Remy]

As long as the thermostat is not stuck in the closed position as it could cause your motor to overheat therefore it could kill your motor causing a blown headgasket, if its stuck in the open position kinda like what you explain (the temp drops) you will be using more fuel to keep the engine in normal operating temps my advice is to change it as soon as possible but as long as your thermostat is not stuck in the closed position you should be able to drive it for awhile you'll just have a lower mpg.