Sheffield UK 15th February 2002

My 1989 740 Turbo estate (it has a 940 2 litre engine) has a very intermittent fault that just will not happen whilst at the dealers - they just can't find anything wrong.
Symptoms are these - Start engine normally and set off - engine stops. re-start and engine fires instantly, maybe runs for two seconds and then stops again. this can be repeated several times until eventually it keeps going and no further problems - sometimes for a week or more.This is not only when engine is cold - can happen when engine has reached normal running temperature. Last Sunday I set off from home with an already warm engine and had driven about 300 yards when it cut out and as I was on a downward slope I tried to jump - restart it but to no avail.Stopping and restarting with ignition key was again instantly successful. I reached my destination and sat with engine running for 5 minutes and then turned it off for a further 10 minutes. I then set off up a steep hill and after 200 yards the engine stopped again. I restarted it on this steep slope about 20 times (each time on the first spark) but it would not run for more that a second or two. In desperation I rolled back into a gateway, managed to turn round and facing downhill got the engine started again. I drove it straight to my dealer without incident and after 4 days of looking at it it has not failed again.
I don't know if its significant but this car is an oil guzzler but that has been so for the 4 years I've had it and this problem has only been around for a month or two.
Any suggestions will be most welcome.