1998 subaru outback continuously stalling


My girlfriend has a 1998 Subaru that recently blew a head gasket. We took it to a reputable shop and had the gaskets, seals, timing belt, and a basic tune up performed. The car currently runs fine but now it is stalling erratically when it comes to a stop. Also, when it is driving at higher speeds on the highway it will begin to show some hesitation when attempting to accelerate.

To fix the problem we also had the intake manifold sensors, engine computer, air flow meter, cam and crank sensor, alternator, fuel pump assembly and coil pack replaced, free of charge by the original mechanic, but the problem still has not been fixed.

My girlfriend and I are both financially limited college students and are stuck in a tight spot with needing an operational vehicle. Any advice on what the source of the problem could be would be greatly appreciated!!!