I suspect all your filters including the one in the tank are fine. You were almost able to maintain 80 mph for an hour. Engine quite only when you reduced speed drastically basicaly. Easiest test I can think of is to listen when you remove fuel cap after driving awhile. If tank is under quite a bit of vaccum you should hear it. Again the only variable I can think of is the ability of the lift pump to pull fuel might have been reduced when revolutions dropped on 123s and perhaps on 115s as well. Partially obstructed and totally obstructed tank vent lines are fairly common. The other thing that will shut these engines down is excess blowby. The crankcase pressure shuts the injection pump off. If engine was quitting at speed a guy might want to make sure his engine vent or pvc line was clear. But you ran fine at speed and I cannot see suddenly developing excess crankcase pressure when you slowed. The ability to restart pretty quick also leads in the first discussions direction.