Thanks again,
I think I've narrowed it down to the ignition system. Checked socket #6 and get a 3-1-4 code (NA according to chart) and a 3-2-4 code...not on the chart. Optimistic as seem to be narrowing in on the problem, but frustrated at not finding THE solution. Called Volvo before finding this code and they seem to think it might be a relay problem. They suggested giving a relay near the coolant tank a whack to see if that would prompt a restart and hence locate the problem's source, but there isn't a relay near the coolant resevoir. There are two in front of the radiator unit itself. Perhaps they were referring to one of these.