Do you have the owners manual? Have the recommended items been done for you age/miles VW? If all normal maintenance has been done, then it's time to take it to a trusted mechanic (google 'mechanic finder', there's one at cartalk). It could be an issue with the ignition system, the coils were known to fail on that vintage VW.
Is the 'check engine' warning light (or any other warning light) on?