took the cossie out this afternoon, and todays fault is the temperature gauge

Sitting in traffic the gauge reads normal, but when driving the thing it barely gets off the bottom, and if I drive it from cold, it barely moves at all until it stands still for 5 mins. When I get to a giveway, the revs are sitting at 1200rpm (cold idle) once the gauge moves up, the revs drop to 900rpm. Now im thinking the car thinks its cold and is raising the idle. Its started since I fitted a thermostat from Burton Power last week. So im thinking the stat is stuck open and isnt holding any water in the head??

Anyone had this before??