revs all over the place?

wierd random fault..driving along suddenly car revs up and sticks at about 2500rpm, and then drops back ..sometimes it doesn't drop back.

okay for a week or so, then couple of days probs, and can cut out when I stop...then it's okay again for couple of weeks....also note the water temp stays quite low..I got a slight leak on thermostat, but nothing else obvious.