My car is a 2005 Subaru WRX Wagon. It has 120 000 miles and has had all the regular servicing. Last night I did some highway driving and then hit stop/go traffic. After about 10 mins in the congestion the revs started to increase when idling (clutch disengaged and in neutral). Typically it sits at about 600rpm or so when idling, it was started out at 1500rpm. After another 5 minutes it had passed 2000 rpm, another 5 and it was close to 2750rpm. Felt as if I was holding the accelerator down constantly. I didn't have any engine management or oil lights showing on the dash, and temperature wasn't any higher than normal. This problems has been occuring whenever I get stuck for a long time in traffic and then goes away as soon as I can drive at normal speed. I suspect this may be related to the temperature rising under the hood. The car is not drive by wire.
Coolant temperature was logged at 212F when this issue was happening yesterday.