Had the same problems on a MK3 P reg 8v, used this forum to sort it out. Changed the rotor arm, and the ECU relay to no avail. My mate's dad runs a garage, he cleaned the throttle body out (for ?50) and it hasn't cut out since. One garage I spoke to before this tried to get me to pay the diagnostic charge and wanted to replace the throttle body itself, which they said could cost up to ?600.
From what I can see, the best option is to get the throttle body cleaned if the rotor arm and the relay don't work, the arm wasn't that expensive and I got the relay for 8 quid. At least the body-clean is cheaper than some of the options, and seems to be the #1 solution. Thank God for this forum, I was going wappy with the damn thing cutting out all the time.