Got to be electrical, have you checked plugs, HT leads, connections, "coils" etc, etc.
What's the age/mileage of your 748? If it isn't any of those do you hear any unusual noises? a clunking/knocking noise? do you know, can you tell if the l/h engine casing has been removed when serviced? you should be able to see new threebond (SP???) on the casing/seal etc, the reason I say this is that the alternator nut may be loose which can cause a similar problem, it should be re-torqued every 12,000 miles I believe, but the casing needs to come off to get to it.
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