You may have a fault with your throttle body actuator. When the position sensor starts failing they open the throttle slightly more than normal which gives the high idle. The cruise control won't operate either because it can't work out where the throttle position is.

You'll need to get some fault codes to be sure though. I'll try and find some tests for you tomorrow.

__________________
Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32