Car idling too high

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Late last night after a long blast thru the hills, Im pushing the car pretty hard. Its cold outside I want to make the most of the cold weather. I noticed my car is idling WAY too high. I realise Im having to stomp on the brakes harder than usual. Kind of disconcerting.

I pull over and put the car into neutral, and tacho shoots up to 3000rpm and sits there, revving. I turn off the key, and let it sit for a little while and start up the car again. This time its revving to 1500rpm in park. What the...?

I take it easy getting home, park it in my driveway all the while the car is idling higher than normal, turn the car off, then turn the car on, and its still revving over 1200rpm, but this time I notice my temperature gauge not working, so I popped the bonnet, then all of a sudden...., the temp gauge works and the revs die down back to normal. Im sitting there scratching my head.

I grab the flash tuner and proceed to 'reflash' the ecu in an attempt to wipe the memory clean. I start the car up, and its improved, but still idles high. I leave it till the morning wondering what on earth started this up.

This morning, I decide to check the car by starting it up and seeing if there is any difference, seemed to idle ok. So I take the car for a drive. The high revving returns, but only to about 900-1000rpm in drive, and over 1200rpm in neutral.

I dont know what this is, could I have a faulty temperature sender in the header tank to my radiator? Is the TPS up the duff? Has anyone had this issue before?