The best thermostat to use in the warmer months is the stock one. There are two other thermostats, the tropical one & the artic one as they were labelled by a friend. the tropical one opens earlier & is best for race cars, while the artic one is good for a cold winter, as it opens later. I think there's a 5 degree celsius difference between the three. I agree with Dodgeman, I think that the problem will be a clogged radiator, as the 'rubbish' from the block will have come out & clogged the core. Happened with the B310 coupe with an L18 I own, there was so much junk in there I've had the radiator cleaned twice & I'm still catching rubbish in the filter. Tefba here in Australia sell a cleanable filter, they have a website if you're interested, but there's no room in the top radiator hose for their one. Natrad here in Australia can supply the filter D'man mentioned, the first time I cleaned mine I found swarth in it from the engine. Hopefully getting the triple core will cure the problem for you.