there is one other possible cause.
crank position sensor. a bitch to get to though. down at the front of the block near the flywheel, has a white connector i think. if i wasn't 300miles from my bentley, i'd tell you what the resistance value should be across the pins. this happened to me with my mk3. by this point i was so pissed at the car, i sold it and bought a mk5 but i have been told by the person who bought it that it worked flawlessly after replacing that part (of course )
there are 2 different ones available. they both "fit", one will leak though but you can get some rubber o-rings to get a good seal. of course, the one that leaks is the cheaper one!!!