Stalling in traffic!

My daughter’s 1980 volvo sedan is stalling without any clear reason why. It drives fine most of the time, but will suddenly lose it’s acceleration and need to be restarted whether after a stop at an intersection or while driving on the freeway. Our mechanic has driven it and never experienced this flaw. He repalced the sparkplugs, checked the fuel pump, and can’t find anything wrong. What can I ask him to check out to help get this fixed???

When was the last time that the fuel filter was replaced?
If it has been more than 15k miles, this might simply be a case of a partially clogged fuel filter.