Well, reading all 31 pages

it seems pretty obvious that there are a number of different causes for this problem, It is not always helpful when posters jump in telling everyone that (whatever fixed it for them) IS - in fact - MUST be the solution!
Here's my bit to add to the confusion:
Had problem similar to original post (note now that some are complaining of different problems to the thread title and there are at least 2 completely differing faults being reported in this thread - probably more).
Just to clarify my fault - 8 months ago car (8v GTI '97) cut out after thrashing it a bit, approx 10 mins into journey after starting from cold. Tried relay 30 (as I already had one in car being aware of the issue) - no good - stuck on motorway, got towed in to VW dealers - changed coil - fixed problem and worked fine until 2 weeks ago. Began cutting out under similar circumstances -10 mins into journey after hard driving - (I'm straight onto Motorway). Did this on 3 or 4 different occasions - just died but all electrics OK and car cranked over fine but appeared to have no spark. Left for a few hours and all but the last time started fine afterwards - BUT the last time wouldn't start so had to leave it in a carpark - breakdown driver sent for it next day he said it started fine. Told him to take to dealers anyway for check. They said runs for 10 mins then cuts and it IS the main ECU (not the relay). Over €1000! I told them to first rectify leaking coolant from where top hose meets engine (by temp sensor) and also leak from joint of newly fitted exhaust system. Also to confirm with VAG computer that it is definitely the ECU.
They said yes yes etc etc. Just got it back - they HAVEN'T fixed any of the above so told them to stuff it and I'll be claiming for a refund of the 3 hours labour they charged me - as well as passing this on to VW Spain. They gave me a discount but also flattened my brand new battery! I am going to fix coolant leak (it seems obvious to me that as this sensor feeds back to the ECU that this could then send a STOP signal to protect engine from oveheating). I will also check the fans - which I haven't heard come on for some weeks even though outside temp is over 40 degrees and the engine is always really hot when it cuts. This is something else I asked them to do but they "knew better".
If it's still not working after this will look at getting a used ECU for €65 and put it on myself.
Will post back.