[E.D.75GMC]

It's all sorted now and the truck is running well! We played with the timing a bit, got that adjusted properly. Funny thing now; the timing didn't fix the issue. When we had the hood open while the truck was running, right after taking the truck for spin to see how the new timing worked out, I could faintly smell burning plastic. It was coming from the distributor. Shut the truck off, popped off the cap, everything looked good. Popped the plastic piece where the ignition module sits and BAM - there lay the problem. Turns out the module was slowly frying itself, and the plastic casing was warped and partly melted. It must have been cutting power to the spark plugs and that's where the stumbling was coming from. Replaced the module and went for a good 40 mile drive and it was 100% perfect. Very happy to have it solved now, and thank you to everyone who commented here. It's always great when people help each other out. One of the many reasons why I joined this forum. I hope this thread helps others out in the future!