Well my peugeot problem was found to be a bit of circuitry that is next to the throttle cable in the engine.
It's housed in a plastic box near the throttle cable, it has a self adjusting thread with rubber around it to keep it sealed and it should move back and forth depending on how much you rev the engine, it keeps the the car from stalling when idle, and also keeps the revs constant and not erratic.
I wish i could remember what it's official name is, well what ever the name is i had it replaced by the garage and it's been alright ever since (Touch wood)
To find out if thats what maybe wrong with your car, start the car, open the bonnet and look for what i described above and then pull the throttle cable you should see the valve (Thread thingymejig)
moving back and forth and if it's not moving then thats probably whats wrong.