Back again... I would check the fuel pressure and delivery. Use a gauge attached to the fuel injector rail {tube connected to all injectors... with attached fuel pressure regulator} where there is a schraeder valve, just like on a bicycle tube. Screw on the connector, and turn the ignition to Run. The pressure should build immediately with no other action necessary. Turn off the power, and the pressure should remain near 40psi for 1/2 an hour or more with no drop. If it drops, you have a leaky check valve, injector(s) or fuel pump.
Check delivery using the 'relief' valve and tube attached to the gauge. Turn the key on, and catch fuel for a known period of time and then measure output. Do some math, and figure if it is enough to run the engine at highway speeds. If it works out to over 2-3 gallons per hour, that would be enough to run it at 60 while getting in the 20-20+ mpg range...
Other than that, I'd be checking for good grounds and tight connections, and an ignition switch that wasn't worn out. They can start to separate, and your engine will just stop.
tom