Hi Howard:

This sounds more like a failed Fuel Pump Check Valve issue to me. I had the same issues with my car several years ago, and the problems went away when I replaced this $16 part. Available from FCP Groton, it does not take long to install.

http://www.fcpgroton.com/product-exec/product_id/5147/nm/Volvo_240_Fuel_Pump_Check_Valve_1985_1993_Bosch_/category_id/149

It's easy to check the fuel pressure regulator under the hood, just remove the vacuum line, and check for gasoline or a very strong gasoline odor. If that's not the issue, then I would check the check valve on the fuel pump. Did your new fuel pump come with a new one, or did you re-use the old one?

The check valve keeps fuel in the fuel line so that you have full pressure when you start your car. If the valve has failed, after 30-60 seconds, your system has been re-pressurized, and all is back to normal, so no stalling.

Be sure to follow up and let us know what you find out.
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If it needs to be maintained, repaired or replaced on a 1990 240, I've probably done it. '90 240DL, 323,320 miles, will she make it to 400K ?? >>You haven't really worked on a car until you draw blood<< :-}