yeah the mk3 16v golf gti has regular probs with relays or the ecu fookin up, not sure on the spec of a 1.6 gl as to what kind of igniton its running, i take it its fuel injection so could possibly be a similar fault, the reason your oil light comes on is because when your engine dies the oil pump is no longer turning, hence to oil light coming on as you have no oil pressure and seen as youve changed the switch for that and its still doing it that kinda eliminates that anyways. have a look in the mk3 section and see if you can find out which relay it is for the ignition/ecu then see if you can borrow/swap one with some one with a similar model of golf, cause trying to sort these probs can cost you a fair few quid, so its handy to try bits off some one elses car that you know is ok, then if you dont have the fault go buy a new version of the part youve borrowed.
is the problem only happening when you pull up at a junction etc and the car should idle on its own?? casue the gti prob tends to happen at any given time which can be quite dangerous.. if it is only at junctions etc when the engine revs drop down to idle level, try a few simple things first, change your air filter (£4 ish), and whilst doin that check no pipes from the air box to the engine have any spilts cracks, as any kind of leak on the induction side would also have this kind of effect.
let us know how u get on
