Ok here goes - it's that time of year again - outside temperature is ranging between -2 on a good day and -10 on a bad one for the last two weeks and yet again my temp gauge is playing up. After reading several threads my understanding is that when the thermostat is working correctly it should with a bit of give and take read 88 degrees regardless whether I'm going uphill/downhill, urban/motorway driving, heaters hot/cold and any blower speed and furthermore whether I'm moving or standing idle. Please correct me if I'm wrong. Well anyway, last February, after noticing my temp gauge took ages, sometimes hours, to warm up and then only hitting 75 degrees I changed my thermostat. What a difference! Internal heaters remarkedly improved and the car running at the aforementioned parameters. Well this lasted for 5 months and then I noticed whenever I went downhill the temp gauge would start dropping. The further downhill I drove then the lower the temp would go. If the hill was long enough it'd even drop to zero and living in the South Wales valleys there are quite a lot of hills. Well fair play to my local Vauxhall dealer, who unlike some are out of this world, they offered to replace and fit a new thermostat as it was guaranteed for 12 months even though it was originally fitted by a mate. Bingo! Job done, everything was working great untill about a fortnight ago when I noticed that if I was standing idle ( with the heaters on no.3 setting which is my norm. and bear in mind the current outside temperatures ) the temp gauge would slowly drop to zero. I should add that I'm a taxi driver and with the current economic climate I spend a lot of hours sitting on idle. Well I automatically assume thermostat must be dodgy again. Fair play, off to the dealers, bob's your uncle, new thermostat fitted and disaster. Not only does the thermostat operate at a lower temp, say 84 degrees, as well as having slightly cooler heaters, same symptom of temp gauge dropping when idling, the car now loses temp when driving downhill. So I inform the dealer that they must have had a dodgy thermostat and also when they fitted it they hadn't put the egr cooler back properly as it was hissing exhaust gasses which could be visibly seen and heard. No problem says the dealer ( see, I told you how brilliant they are! ) we'll order a new thermostat and try again and refit the egr cooler pipe properly. That was 2 days ago. Yesterday, they fitted a new thermostat, sorted the leak and guess what - exactly the same problems. Now I know it was definitely changed because cynical ****** that I am - I marked the old one. What do I do? I think it's a dodgy batch of thermostats myself but if it's not and they've fitted two which would suggest that it might be something else then what could it be? I'm apprehensive about going back a third time but am at my wit's end. Opinions and suggestions required please. Bear in mind it's a working car and just in case you think I've forgotten to say what model it is - I haven't - it's a 55 plate 120 bhp 1.9 CDTI Vectra with a Z19DT engine. Thanks in advance.