I'll bet if you pull off your battery cables you'll find white corrosion under at least one of the clamps. I've seen them completely rotted away on the underside of the clamp. Also a clamp could be partly cracked through, giving a poor clamping force on the battery post. By jumpstarting you could be moving things just enough to get a good connection for a while. This is even more likely if you have replacement battery cable ends- the Pep Boys type of thing with a two-bolt clamp for the wire connection. It's something basic like that. By the way if the insulation is cracking or coming off the + cable, it's time to replace it NOW before it causes a fire. My friend lost his '88 740 from that exact cause. Very ugly and almost impossible to put out once it starts.
Good luck!
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Rob Bareiss, New London CT ::: 86 244DL, 87 244DL, 88 744GLE, 91 244: 808K total