Car keeps stalling and sputtering out - nobody knows what's wrong!

I have a '98 Mercury Sable wagon, over 100,000 miles. For the past 4-5 months, without warning, when I start it, it will stall, and sputter out. It’ll start a couple of hours later, and run like there’s nothing wrong with it. It’ll then happen again in a few weeks. The only pattern I see is that it happens on days when I’m out and about running errands, or taking short trips. One garage said there’s nothing wrong. The dealer said fuel pump but I had that replaced in '04. Yet another garage said not fuel pump and doesn’t know what to do? Anyone out there want to try to diagnose this for me? Thanks. April

You need to attach a fuel pressure gauge, a test light to attach to the fuel injector wires, and get some way to check for spark, like a clamp on timing light. Then, when it stalls see which of these things is keeping it from running.