Easy way to find out if it's the thermostat, next time it gets hot, feel the top and bottom radiator hose; if there is a major difference in temp between the two, you may have a stuffed thermostat. I once fitted a new Tridon brand one in a Bluebird I had... drove a few kays and I could hear the coolant boiling in the radiator. turns out it was jammed shut from new! Took it out and drove and was able to get it back home although alot cooler than it should be.