Temperature Gauge Fluctuation
 

I have just bought this 1998 Sebring LXI and I wonder if this is normal.

The temp. gauge goes up after start up and stays at the middle. Then in the highway it goes up, close to H mark and then it comes back down close to middle. This cycle continues through out driving. Is this Normal? And if not where is the problem?

Assuming the coolant level is full......

That is CLASSIC head gasket failure....

I have to disagree Djinn,

Car idles smooth, there is no smoke, oil looks normal and clean, car just passed smog and there is no check engine light.

Compression leak into water jackets.....

At Idle , car runs fine, accelerate or highway cruise, temp climbs....

Combustion pressure overcomes cooling system pressure and blocks flow....Temp continues to rise until you let off....

Poor running, smoke and milky oil are the symptoms for coolant leaking into oil, not the combustion gasses leaking into coolant....

But I dont let off, I continue going at the same speed, the temp. gauge reaches a certain high point and then comes back down on its own, then the same cycle continues.

The temperature gauge can fluctuate very rapidly if the sending unit is not in contact with coolant. This can occur if coolant is low or if gasses are being injected into the cooling system due to a blown head gasket.
Check the coolant level first. If it's low, find out why.
Pressure test the cooling system and check compression.
Alternatively, keep guessing.

Mechanic shop ruled out head gasket promblem.

They suggested changing the radiator cap which I did, now it's somwhat better but not resolved completely. I plan to change the thermostat and the sensors.

Will follow up here later.

*** SOLVED ****

Changing the radiator cap helped a little bit, but changing the thermostat solved the problem completely. I drove the car for two hours on highway and the temp gauge never passed above the half mark.