[Volvofreak2]

Had the same problem with my 850 20V. Read your thread yesterday and, after work, went to my garage, to check, if it starts, when connecting a charger, as you described. When connecting the charger, I found out, that the nut, that holds the two additional wires to the positive batterypole, has come loose. Tightened it again and gave it a try. What should I say, engine started at once without problem! Same today! I would say, job done! This might not solve your problem, but shows to me, how simple failures can cause interferences and should be checked first.
And thank you very much, for leading me in the right direktion, I suspected a fuelproblem!