Simple as a sticking thermostat?

Calling the Griff doctors. I'm in Spain at the moment for the MotoGP and my car has developed a slight problem (it always happens when you're away!). Normally I have no cooling problems although my gauge perhaps under-reads. It usually sits at an indicated 80 after going slightly over, then the thermostat opens and it rests at 80. The highest indicated, in hot ambient conditions in slow traffic was approaching 90. It's now going up to 80ish then dropping to 70 or just over whilst cruising. Just the thermostat jamming open?

Thanks.