Thank you very much for your detailed response. The spark plugs were replaced yesterday as well as a coolant leak repaired (engine has been hot after 9 mile commute drive home, uphill, recently). The Volvo stalled five times on the way to work this morning, mostly on the flat after a one mile downhill portion - as the road turned flat the first stall happened, then it happened again in several minutes, twice in a short time, then again a mile later. I drove immediately to the shop which had done the previous day's check, and two of the mechanics drove the car during the morning. One of the mechanics did everything he could to replicate the stalling, including hard right turns, and neither one experienced the engine dying.
Their immediate suggestion is that I let the car warm up for five minutes before starting in the morning, as my first driving is uphill for three blocks before starting down a relatively steep one mile drop to the flat portion of my commute. I do not normally "warm up" the car as I was under the impression that this is not necessary for relatively "modern" cars. The mechanics said that the car is getting old (146,000 miles) and that it in fact needs time like people getting older to warm up... rather folksy of them. However, my last Volvo (1986 740 GLE) never needed that and it lived to 450,000 miles.
They also asked what kind of fuel I use - which is normally Chevron, used to always use Supreme but now go for the middle range fuel. I did notice several times after refilling a nearly empty tank that the car had maybe two instances of seeming to "cough" or stutter, so I never let the tank get much below a quarter full. My feeling is that I should drive with the tank above half full and probably switch back to Supreme (or whatever the high octane gas is).
Tonight I drove the car home after warming it up for five minutes and had no stalling. I do not know of course whether the mechanics tweaked something today while they were attempting to find something or the warm up and fill up were the answer. After stalling five times today on the way to work I am anxious to see how it performs tomorrow with this new scenario.
I will keep your comments at hand for the next time I talk with the mechanics, which will be to either tell them their suggestions have solved the problem or that they should work through the suggestions you provided. According to the mechanics they could find nothing through the computer or their physical checks. They didn't charge me anything for their recheck and test drive times today - good guys.