You ever figure it out man? Did the Sea Foam help?Just a follow up to what you were saying about the valves on cylinder 5. Logically, if it had a bad valve spring or bad valve, wouldn't I be floating the valves and losing power at WOT and higher rpms? When accelerating hard from a stop with my foot to the floor and letting the truck wind out every gear it usually does fine, or won't throw a code until it shifts to 3rd or overdrive.
I did have to have the heads rebuilt. sticking valve. lots of carbon buildup on the valve stems, under load they get hot and swell and stick with that extra carbon on them. Head rebuild fixed it. Recommend you don't buy new head or Ebay heads. take your originals to have them rebuilt. Don't throw parts at it, let a good mechanic do a road test with a vacuum gauge hooked up. I sunk close to $6K into chasing my problem. Don't do what I did. change out your spider injectors out to the newer style MFI system while you are in there.You ever figure it out man? Did the Sea Foam help?
I have identical issues as you except its cylinder #3/#4 and a P0300 for random missfire.
New parts include: Metal distributor, cap/rotor, platinum plugs, NKG wires, Coil pack, Ignition control module, MAF, Crank Sensor and recalibrate, Fuel pump a few years ago "ITS LOUD"
Fuel filter recently, Reman Spyder injector a few years ago as well as intake manifold gasket, compression tests good, never checked valves yet. Cats gutted, new 02 sensors, tested fuel pressure.
Vac tests and lots of scans, always cylinder 3 and 4 missfires, mostly #3. I DRIVE THE SHIT OUT OF IT and it doesnt help hahaha.
Loses power on the same hills, always starts out good then starts losing speed, downshift, downshift, downshift, starts shaking, flashing CEL, have to slow down to 80kmh for it to stop shaking and then its good until I get on it again and try to maintain speeds above 100kmh on big hills and summits.
unfortunately out of all of the 4 different mechanics, none of them suggested it could be the heads... Well looks like I know what Ill be doing.Stop throwing parts at them guys and just have the heads rebuilt. It DID fix my truck finally and I should have started there and not wasted the extra dough. Although not an easy job to shade tree, and expensive at the shop, I think this is the root cause of many "mysterious" misfires you can't fix with electronic parts. Dirty, sticky valves will not be helped much if at all by seafoam and treatments etc... and driving the piss out of it only made it worse cause it got hot.