>This is a tough one to debug because the problem never occurs during idle 
>or engine revving and only randomly on the road...
>
>I've already done full tune-ups and replaced the fuel and air filters.
>

Check the double relays, as they are capable of switching off the entire FI 
system if they lose power.  Maybe they are dirty or corroded?  Perhaps under 
acceleration the wiring harness gets tugged slightly as the engine twists, 
causing a bad connection to break momentarily?   Tim