Stalling

My ’94 Volvo 850 wagon is recently experiencing an intermittent stalling problem. It’s an inline 5 cylinder with fuel injection (no turbo) and it has a manual 5-speed transmission. We’ve determined that it’s a fuel delivery problem. Fuel pump was replaced 3 years ago and appears to be working properly. Have also determined it’s a problem with power delivery to the fuel pump. Installed a buzzer in front of fuel pump; when power is going to pump, it makes an annoying sound. Goal was to see if next time car stalled what would be state of buzzer: if stalled but continuously buzzing, something is wrong with pump; if stalled and buzzing stops, then something is wrong with electrical delivery. Buzzing stopped at next stall; so we know power was interrupted. If no buzzing, car won’t restart; however, as soon as buzzing is heard, the car starts and runs. Ignition switch seemed a bit loose and mechanic replaced it without benefit; so, returned it and reinstalled old ignition switch. Mechanic says fuel pump relay tests okay, but how might I test power to the fuel pump relay... Install buzzer upstream of relay, or is there a simpler test? What are power systems upstream from fuel pump relay? My mechanic says there are no computer error codes, but also tells me that error codes in that old of a car aren’t very exhaustive. BTW, It happens in cold or hot weather, whether humid or arid; it is also indiscriminate as to the operating temperature of the engine. It’s pretty frightening for my wife to have her car stall on freeway and have to coast off to side of road until it will start again. Any diagnostic thoughts? Thanks. Jeff C