Hi,
well it can have many causes 8) , lets start with the vacuum piping, is it OK?, are the contacts all in order?, is the system pressure in order?, ist the petrol pump OK?
Theses are a few standard faults, it seems that when the car stops the petrol shoots forward, the question is where and why!? I've never heard of this particular problem before. Check your system over before you decide to give out money!
Dave