John, I hope it's not this but I had exactly the same symptoms on my Croma 1.9 8v late last summer, the car would start to show a slightly raised temp guage reading, this was intermittent at first and would be fine for days then would play up for a few more then go alright for a few days again. Eventually it got bad enough to put the 'Excessive coolant temperature' warning up, the engine did not feel as though it was overheating though. Classic sticking thermostat I thought and went out and bought one for £63 from my local dealer (includes the housing & temp sender), this was a bit of a sod of a job but completed o.k. This did not cure the problem however, I got suspicious when I put the heater on as the weather turned a bit cooler, it was useless and blowing cool air. I finally tracked the problem down to the impeller having come loose on the water pump shaft. The impeller is plastic and pressed onto the shaft via a cast- in bush, when I removed the pump I could spin the impeller on the shaft. This is a common problem with cars of the mid 2000's as it was another cost down exercise. I went to my local motor factor and bought a new pump for £27...with a metal impeller, dread to think what it costs at the main dealer and fitted it with no further problems since. By the way, the bearings on the old pump were on their way out as well, this was at 46,000 miles so I changed the timing belt while I was at it. I wish you the best of luck.