high speed miss

[goodstuff]

I have a 2001 Express 3500. It runs like a top to 45 mph. At 50 some shuttering which I assume is lock-up. Anywhere from 55 up I get an occassional misfire. Sometimes once, sometimes 3 or 4 in a row. Have changed rotor cap wires plugs fuel filter. Fuel pressure at 55 psi. Snap -on scanner shows misfires on cylinder 5. Need advise. Should I replace one injector/popit or go for the new injector upgrade, or am I on the wrong track. Van has 97k. Thanks

[fliz]

I'm bumping this because I'm having a similar problem, and can't start new threads yet.

 

Anytime I go over 65mph, the CEL flashes. I borrowed a scanner, and it showed P0300, random misfire. There are no symptoms, so I'm thinking maybe it's a sensor problem?

 

It's a '03 Express 2500 van, 6.0L w/ auto trans. 105K miles