1992 Oldsmobile Cutlass car stalling out

1992 Oldsmobile Cutlass 6 cyl Automatic 100000 miles

Hi,

I'm in need of a little guidance. A friends car (described above) will; warm up fine, drive (city) for 15-20 minutes then just die wherever it's at. After 5-10 minutes, it will restart and be fine for awhile, then die again.
When it is running, the engine sounds really good, no wierd sounds or idling patterns. No warning lights on the dash. With the engine shut off or running, there is fuel under (some sort) of pressure as evidenced by removing the protective cap on the fuel line inline bleed valve positioned beside the fuel rail and pressing on the valve stem.

They have had the car about 6 months (bought used, obviously). I'm unsure of the mileage on the odometer. The car quit on them last Saturday. Cost them $60 for the tow. Cost them another $225 for a new fuel filter, a new sending unit for the (radiator) fans, and a shrug of the shoulders from the mechanic.

I, haven't laid a wrench on it yet as I would prefer to get some knowledgeable feedback first. My skills are pretty good, my tools (diagnostic and all) are 350 mile away so I'm a little limited, but if it's fixable by backyard mechanics standards, then I'm pretty sure I can handle it.

Any help would be appreciated tremendously.

Thank you in advance,

Jethro B