Overheating!!!

Hi,
I live in England and have a 1999 Explorer. I love the car, but I have just started having problems with what appears to be a overheating issue. The temp gauge goes to the red pretty quickly and only goes down if I have the heating on full, so have to have the windows down. I have had the temp sensor replaced (I was told it was the problem) but it's still doing the same thing. Any suggestions/help will be appreciated. Thanks.

replace the thermostat and have the fan checked

I did wonder about this too. Hope it cures the problem. Many thanks.

Now I've also been told by a mechanic that it may be a problem with the temp gauge in the dash panel, as the needle does not return to normal after the engine has been switched off. It stays wherever it was last situated, normally near the red. The car doesnt appear to be overheating, no water leaks or steam at all, it drives fine too.

I was having, what sounds like the same kind of problem with my 99 explorer. I ended up taking the it in and having the radiator flushed and putting new coolant in. My friend worked at the dealership so he ended up doin a super clean on it, and now its running strong. Did this over a year ago and haven't had a problem since. Hope that helps.