Mr2 Over Revving... But Only Sometimes.

[hjs]

hi, iv looked about on here for a while but could not really find anyone with same problem as me.

iv had my mr2 mk2 for about 3 weeks now and its been fine. until a couple of days ago. i drove to hull, and it was about 100 mile round trip. engine was completely fine and was warmed up when we go stuck in traffic. i noticed my rev counter said it was doing 2000 in idle. when i press the accelerator it goes back down to 700-1000 revs which is normal. it doesnt stick to 2000 all the time in idle. but when it does its a perfect constant, as if i were driving at a steady pace!? can anyone shed any light on this? i have no idea what it could be?