I have a 1986 gmc 4x4 3/4 ton high sierra. Just got finished restoring it. However the engine is out of an 81. I don't know much about the engine other then it looks clean and runs really good and strong, well it did. The other day when I was driving it, it acted kinda funny. I want Opinions besides mine. While in first gear shift to second and if you accelerate normally runs just fine but when you lug the Engine it seems to me it's missing or something while accelerating pretty hard, it starts to go away soon as you let off the accelerator. If you mash it to the floor while it is doing this it almost acts like it wants to kill it. But just driving around normally it seems fine. But when you mash the gas while in gear or hot rod it a little bit it acts up. It's a manual btw. I think I know what it is causing it, I want some opinions to reassure me. Thanks