Cooling system problem??

My car's been having some problems as of late.

its a '98 sl2 w/ 99k mi.

For some reason after driving for 1/2 or so my temp gauge starts jumping around causing my engine rev up and down. After doing that for a couple of minutes the gauge will jump way up and the Service Engine Soon light comes on and then the gauge goes back down to normal (at least for a few seconds, until it starts jumping around again). I have sufficient coolant in the car and i have no leaks, although, i don't seem to be getting enough pressure in my cooling system since i can pretty easily squeeze the hose that connects the engine to the radiator. I think that indicates a pressure problem.

I've checked around and it sounds like it could be one of about a thousand different problems. I'm just trying to get it narrowed down a bit before I start spending money to fix it.

Some of the problems I've heard of might be:

1. bad radiator cap

2. bad thermostat

3. bad temp sensor

4. clogged hose

5. broken fan

or a combination of any of the aforementioned.

Has anyone had the same problems?

I dont want to go out and start spending $$ on something that may or may not fix the problem.

You have a bad coolant sensor more than likely.

Derek
PA