[Chimfender]

I have a an 02 KV6 Conny SE Auto Saloon.
Always runs fine except yesterday. Stopped at the traffic lights with lever in D engine ticking over at normal idle speed foot on brake. Suddenly the engine speed slowly increases to around 2000 rpm then comes back to idle. I resisted the car moving by increasing the foot brake pressure. It then happens again 2 or three times. I then notice rhat when it happens, curiously the water temp gauge needle falls back to zero then goes back up to normal when the revs drop. Driving away when the lights change the engine appears to be ok and the fault appears to have cleared. My first thought without to much knowledge was throttle pot but how would the water temp gauge movement be related it seems crazy. Any ideas would be appreciated