97 Cavalier 2.2 Heat Gauge Problems??

I have a 1997 2.2 Cavalier and I recently had the head replaced because it was cracked, before the head was fixed the heat gauge would go up about 3/4 of the way but never in the red but now the gauge is going in the red when the car is ideling or driving at low speeds but as soon as I get it up to around 35 the gauge drops almost instantly. The fan is working fine it kicks on at around 200 degrees I also had the thermostat replaced twice and it's working fine. I have had it checked by 2 diffrent mechanics and they both hooked it up to a computer both times and both times it wasn't regestering hot when the gauge was in the red, when it goes in the red the check gauges light comes on but not the engine or coolant light. I am afraid I am going to end up replacing the head again driving it while it's doing this. The head I had put on is a brand new head and the compression is good and there are no leaks. I am about at wits end with this thing. Is this a common problem with these cars? Should I be worried?

Thanks

I'd like to know also. My 97 Cav has similar overheating problems, except mine starts with the oil pressure light coming on and the thermostat heading north. I had the thermostat and the oil pressure sensor replaced, and have a new motor in the car, and the exact same thing keeps happening. Is there an ECU in these cars? I've seen bad ones wreak havoc in other vehicles and was thinking of replacing that next.