Volvo 940 GL Sedan with B230F engine. LH2.4 & EZ116K.

In the past fortnight on three occasions, the engine speed has suddenly increased sharply & briefly completely of its own accord while the car is sitting still (say at traffic lights). It feels like tapping the accelarator pedal sharply. It has done this twice when the engine was at operating temperature and in Drive - most recently tonight on my way home from work. I had driven about 25 km before stopping to get petrol. Upon restarting the car I put it in Drive and suddenly the engine speed shot up about 300-400 rpm. It lasted only about 1-2 seconds then came back down to normal again. When this happened, the car lurched forward. It goes without saying, this is quite dangerous as I'm risking running up someone's rear end if I'm in a line of traffic. The other time it did this, the engine was stone cold and in Park.

What could this be? Throttle switch? Idle valve? Something else. I'm going to check the condition of the throttle body this weekend (might be dirty). I'd like to fix this before I have a prang.

Any advice will be greatly appreciated.

Thanks

Brad