Wondering if anyone has ever experienced this problem:

2000 Volvo S40 1.9T
103,000 miles
Recently serviced, new timing belt, new fuel filter, new engine mount.

The problem began in park while idling. The engine began to rev to 1,500 - 2000 RPM on its own and than come back down to normal idle. It did this spontaneously many times. Thinking I could reset it by tapping the accelerator, instead it reved up to 3,000 and never came back down, I had to immediately shut down the engine. At that moment the car did not want to start until it cooled off 30 minutes later.

The problem worsened when it started happening in drive, stopped at a light or rolling the car spontaneously reved and lunged forward on its own for a few seconds and than back to idle. The first few times I had to slam on the brakes to avoid slamming into the car in front of me.

I drove a few miles, parked the car and suddenly the coolant overflowed out of the reservoir spilling out a good amount. I let the car cool off, refilled the coolant reservoir and got the car home. The coolant did not spill over in the 5 mile drive again but the car continued to spontaneously rev on its own and lunge forward and finally the check engine light came on as I pulled in home.

Anyone have a clue what is causing this? Faulty sensor, faulty valve? Could this be an overheating problem?

The timing belt was recently changed (2 weeks ago) by Volvo, could this have anything to do with it? The car didn't feel the same when I got it back, slight chirping sound coming from belts.

I am really lost on this one.

Thanks in advance for your help and appreciate any replies, suggestions, comments.

I had a similar problem on a 440Si (1.8). I know it's a much older car but any how.... The garage changed the carburettor inlet valve and that helped, but then the problem came back, then they did all manner of checks and changes including an emissions test (which it failed)...

The mixture was way too rich, but they still couldn't find a problem and then sent the car to a Volvo dealer and they couldn't figure it either... In the end they concluded that it was probably the ECU.

Make sure that the wheel/tyre sizes are exactly the same on the driving axle, as that can make things go weird if one wheel is turning faster than the other.