Have a 94 chevy k1500 w/ 4sp auto and 350. After warming up (10 miles @ 55 mph) starts losing power, then speedometer and oil guage start dropping and brake light came on. Engine stills runs good. No missing or noises. If I stop for several minutes, turn engine off, it runs normal for a maybe a mile after I restart, then acts up again.
I'd check for a bad battery or dirty cables and it may be losing ground due to that. Check on both ends where it connects to the block as well. Also check the ground wire to the pcm, it should be right near it. Of course this is taking it for granted you have changed the fuel filter recently and fuel pressure.