I believe your '98 has the 606 motor like my '95, only turbocharged (I think). Anyway, I was having a similar problem - you could almost predict when it would die 3-4 minutes after a cold start.

I also had the symptom that when I opened the fuel tank I got a big hiss. Air going in or out I never quite figured out, silly as that may sound.

Anyway, my problem was solved after I changed the lift pump - at the suggestion of others. This is a small mechanically driven fuel pump near the injection pump. Not very expensive and not difficult to change, except for access. I pulled my intake manifold(s). If you go to that trouble, you should also change all the plastic fuel lines while you're in there.

Rgds,
Chris W.