P0305 reacuring code!

I have a 1999 chevrolet suburban with 5.7 engine and 4x4 with 89000 miles that keeps spitting out p0305 misfire codes when under hard acceleration. I have replaced all plugs, plug wires and cap and rotor all to get same #5 cyl misfire code appear. This misfire originally only appeared when pulling steep hills, but now has progressed to even hitting passing gear on flat hiways. I have put injector cleaner in tank and awaiting results if any What is next step inlooking for problem Vehicle runs pefect untill throttle is pushed hard JEG

The Gm fuel injection unit(called CPI) are notorious for going bad, giving cylinder misfires, etc. I would first establish what cyl. 5 is missing, spark or fuel or compression. First start with fuel, it's the most likely. If you have access to a scanner, you can manually trigger the fuel injectors. Do a bleed down test and see if inj. 5 is operating. I've replaced many here. There is a retro kit that relocates the injectors making the engine effectively a MFI rather than CPI.
Sorry it took so long to get back to all of youl

This unit has SFI system with poppet style nozzle tubes. I have been noting that problem comes on at between 65-70 mph and doesn't miss a beat at idle or road speed untill passing gear it engaged. Dealer had swapped out original wheels and tires to later 17" takeoffs prior to my purchase. Could this be causeing any of this problem? I assume it would not only affect #5 cylinder if it did. I am trying to get someone with proper tools to check injector. The 70 mph rpm is about 2100-2200rpm in 4th gear and i have noticed vehicle at much higher rpm's in 3rd gear without and shuddering or misfire recorded. I'm baffled on this cause and have limited resourses and time. Thanks for your input and recommendations! I'll keep you posted!

Tires causing misfires? That's a new one to me. No, the one-offs they put on your truck aren't causing the misfire. That SFI unit you were talking about? It's the CPI module. Like I said, they are notorious for giving INTERMITTENT misfires. I've had one that had 4 injectors air locked after routine intake manifold replacement. The injectors would buzz but no bleed down. After removing the upper plenum and bleeding the inejectors, they were fine for about a minute, then started missing again. We replaced the CPI module with Borg Warner's aftermarket and highly superior MFI retro kit. They''re expensive, but worth it. That's only one possibility.