high speed miss

[goodstuff]

I have a 2001 express 3500 cargo van. Runs great to 55 mph. Above 55 I get a miss, sometimes 1 sometimes 4 or 5 in a row. Replaced rotor, cap ,wires, plugs. Snap-On scanner shows misfires on cyl 5, a few on 6. Fuel pressure holds at 55. I thought pressure may be low but I also have a 1999 suburban that just went 2000 miles at 65 t0 80 that has 50 psi. It runs like a top. Any advise would be apreciated. Thank you.