I'm not sure how clued up the op is but he has been taking it to a garage so I'm assuming they have come to the conclusion it is in fact over heating or else why would they have changed all the parts they have so far?
You would have thought so, wouldn't you. It's a lot easier to diagnose when you actually have your hands on it, we're only guessing here really.
I think the OP is looking for help because the garage is just working their way through a list of most likely causes. It's always been the problem - you pay for work done, not results.
The OP says he has had an obd2 reader on it, the temp on obd is going well over 100°C. However, the garage hasn't changed the head sender, only the one in the rad AFAIK.
FWIW, I never notice the fans run much here in the UK, you have to be stuck in traffic on a moderately hot day.