Bought a 1998 K1500 Silverado Z71 with about 167K on her and the first time I had it on the interstate it seemed fine, but the first long hill the SES light started flashing and the whole truck started shaking. P0305 Cylinder 5 misfire. Just thought it was a bad spark plug or something, so not knowing the vehicles history I changed out all the ignition stuff, new rotor button, distributor, wires, and plugs. Re-routed wires for separation. Still starts shuddering at throws P0305 at speeds over 55mph while pulling hills, worse shaking if towing. Mechanic told me it might be a sticking exhaust valve. We've checked valve lift and that was good, valves are very clean for this year, everything good. Checked compression in each cylinder, all 8 reading the same at around 180psi on a brand new gauge. Decided this couldn't be a damaged valve or ring. Pulled intake and found an intake leak, installed new gaskets and sealant to eliminated that as a potential issue. Still threw the P0305 code immediately on test drive. Checked grounds, cleaned, all good. No change. Checked fuel pressure and did a bleed down test, all good there. I had just put a new sending unit, fuel pump and filter on. Cleaned Mass air flow, new air filter, no change. Took a test drive while my mechanic friend watched the scanner and it was retarding the timing advance under acceleration instead of advancing it. Prompted us to replace the knock sensor. That was the first positive change in drivability. The misfire and shaking is less pronounced, it's not knocking drinks out of cupholders anymore, but still throws p0305 on hills. Decided it had to be a bad spider injector. Replaced that last night and it's the newer MFI style, much nicer, and the old one looked pretty fried on cylinder 5 when we removed it. Was confident we found the issue. Nope! Took it down the interstate and while the timing advance and all the numbers on the scanner appeared to be good, the first steep hill and she started to lose power and was popping out of overdrive a lot and then the dreaded P0305 again. I'm just at a loss here. Anyone know what's going on I'm so tired of throwing money and time into this. It's an incredible looking truck for the year I'd love to be able to enjoy it and be able to tow with it occasionally. Right now all it's good for is sub-45mph around town stuff.