Had exactly this happen recently with our 1998 2.5 v6 and also had one of the dash warning lights come on. Took it to local garage (which fortunately has Vauxhall diagnostics) and this revealed that the crankshaft sensor was faulty. These are small but complex units with some kind of internal coil - when this fails metal expansion due to overheating within the unit causes the engine to shut down. If you wait and allow the unit to cool down the car will work again for a bit, but shut down again when it heats up. A replacement unit is a bit of a pain to fit as it has to be done from above and there is a fair bit of dismantling of facias etc to do under the bonnet before you can get a hand down the back of the engine to where this unit is located. Also, it would seem that there are two types of these sensors with differently shaped fittings (either a square end or an oval end), so you need to have the correct one. Our one was oval, but yours may not be. Hope this helps.
Graham, August 2007
