I have a Fiat Brava 1.4 sx s reg 1999 (i think). for last week or so when car i at idle the tamp is normal...about half way. The minute i start to drive i get around2 - 3 miles if im lucky and the temp drops to cold. Once it does this is starts running slightly erratic as if missing but not. Them fuel ignition system light comes on. If i stop car and restart light goes out and temp climbs only to do the same when i move again. Last night light came on and i was on motorway so had to drive 10 miles with light on car running cold before i could pull over. Pulled over went to restart and revs stuck quite high. light still on. Restarted again and it wouldnt star...as if no petrol getting to engine. eventually it started...although backfired,,,,popped and fizzed and backfired somemore. Ran like rubbish knocking from engine and really irratic. Next day took it for run battery was flat....must have left light on...anyhow running smoth now...apart from over cooling still but if i stop and let it warm up no warning light comes on. There is a slight noise from engine...not a knocking or tapping but somewhere inbetween ...seems to be comming from passenger side somewhere.

Any ideas guys....thanks

Sound like you have a loose connection on your temp gage, give it a bang on top its common. The coolant can suddenly super cool itself unless your fans on permanently.

However sound like you have other problems too, not sure about those.

your thermostat could be stuck open, its worth taking it off to check what temp it opens at (use a pan of water on your hob and a thermometer). it should open at approx 80-90 degrees C, certainly not before 78 degrees C.

2 possible thinks can happening. Or your thermostat stuck open as Jug said or the relay for coolant vent has stuck to the position to making the vent working all the time. You can understand this if you start your engine and the vent start working. If its not then you have a thermostat problem.