98 Mustang GT dying

Start with the basics.
Check all grounds/connections for wiring.
If those check out work on idle related items.
Clean the MAF (with maf specific cleaner).
Clean the Throttle body (with specific TB cleaner). Make sure the gunk around the blade (Carbon build up) is all gone.
Clean the IAC (idle air controller motor).

I literally just spent the last month dealing with this. Intermittent hard starts, car would shut off sometimes but then letting cool down. It ended up being the pickup sensor in the distributor. As it was failing it was intermittent shut off at speed. Eventually died and wouldnt restart.

When it's dying, does the Check Engine Light come on? Even for a moment? And does it ever give a backfire or two?

Even if the Check Engine Light turns back off, the error code will still be stored in the ECU's memory, so go read it with an OBD-II code reader (get one cheap at the local auto parts store). OBD-II will often tell you exactly what's going on. If the engine is starving for fuel, your O2 sensors will pick up on that right away and you will have lean mixture codes. If it's lean mixture, you could have issues with the fuel pump, the MAF, or the O2 sensors themselves-- and lean mixture will also make it backfire sometimes, that's a clue. If your ignition circuits are faulting, OBD-II should catch those too.

I've solved similar problems multiple times on my '95, slightly different system but close enough. One time it was the fuel pump slowly failing. Another time it was the PIP sensor inside the distributor- in that case it would die completely and immediately. Another time it was the MAF. I haven't seen the IAC cause issues like this at speed, I've only seen that causing problems with the idle... but who knows.