the temp guage in my falcon doesnt seem to want to stay steady. as soon as the engine reaches normal operating temp, the guage will range from between the L in normal to the R. now not all that long ago i did the head gasket on it and ever since its been like this. the problem is it will slowly rise to the R then will slowly fall back down to the A then go up again. i havent had much time to look at it but i think it may be the thermostat cos the thermos are workin properly...

Try the thermostat because they seems like its opening too late and when it does open it cools the motor well so it closes. Bit of a C*** but just replace that and it should be all good.

might possibly have air in the system as well.