So much bullsh!t in this thread its unreal. I'm assuming the car has only recently started doing this... and if that is the case then it cant be the timing,unless its suddenly managed to jump a tooth on the pulley...
Theres a sticky all about this above in the maint section...
1.Check your ecu relays and fuel pump relays...These are a common fault on the mk3
2.Clean out your throttle body make sure its not all gunked up
3.Disconnect the battery and let the ecu reset..
4.If you still have the issue then its possibly the crank position sensor, this is located on the front of the engine block, its got a single allen head holding it on.. about 40 quid to replace...