Argh, I just had to replace my thermostat housing again. I originally replaced it about 20,000 to 30,000 miles ago and now I'm having the same problem. Last time when I replaced it I didn't buy this little piece of plastic they suggested because it was 40 bucks.
Just to be safe this time I purchased all 3 pieces and it came to 110 bucks. This is just after having to replace the altenator! argh, so many problems lately