Yeah that really sounds like a sensor problem. Just to make sure you don't have a freak problem next time it drops to the bottom (cold) pop the hood and grab the upper radiator hose, should be pretty hot. In anycase I think the sensors are rather cheap and fairly easy to get to. Might consider just replacing them both if you have some spare moolah. On a side note, if the one the computer listens to is failing also it should come along with a few warm-start issues. If thats not happening it may be safe to assume only the one to the gauge is failing, in which case I think its more of an annoyance than a problem..but I could be wrong.
edited cause I can't friggin type