I've got a newly rebuilt '82 Chrysler 318 in a '34 Plymouth project car. I have a couple thousand miles on engine and started noticing a little roughness at idle. At part throttle the stumble is more noticeable, but not steady. I noticed that all header pipes are a little discolored, except #4 & #5 cylinders. With a digital temp. gauge at idle all cylinders read 300 to 400 degrees, except 4 & 5, which read around 185 to 200. I swapped spark plugs with other cylinders and nothing changes. I swapped plug wires, still nothing. Everything is brand new, aftermarket electronic distributer, accell 8mm wires, plugs etc. I did a compression test. All cyl. are good. Leak down test, all cyl. good. Pulled valve covers thinking maybe I had lost a coulpe cam lobes, but all rockers arms are moving properly & no bent pushrods. I have a dual plane Weiand manifold with 600 cfm Edelbrock carb. I am really stumped at this point. The only thing I haven't been able to swap is the distributer. I have an in-line spark tester and it shows every plug is firing. HELP !!!