I got this old truck out of a farm field several months ago, and have finally got it running pretty well. I have replaced the fuel tank, lines, carburetor, cap, rotor, wires, points, plugs, condenser, coil, engine oil and filter. I've checked the compression dry and wet. It had 2 dead holes, but was able to get compression back with a valve adjustment. It starts and idles/revs really nice. has 21 inches of mercury vacuum at idle. it does not have any cooling problems.I can drive around the block 20 times and it won't stall or die. on trip number 21 or if I accelerate hard, it usually stalls or stumbles/run rough. I'll pull over, open the hood, look things over, start it back up and it idles fine, revs fine. another 20 laps, and it may or may not die and restart. on one of these trips I found the through connector on the firewall had two pin connectors rotted completely away. I fixed them by just cutting and jumpering with wire and butt connectors. I also replaced the battery cables and cleaned/tightened the body to engine ground. I have 9 volts through the resistor wire to the coil when running. I tried jumping a full 12 volts straight to the coil and driving. didn't make any difference. Sometimes, not always, at idle there's a pop or light backfire through the exhaust. .I also checked for timing chain slop while I was adjusting the valves. How in the world can an engine with 21 inches of vacuum that idles nice and drives nice all of the sudden run like crap and then just as suddenly run fine again?  The engine has a cast iron 4 bbl intake, new Holley 600 carb and old rusty headers with glass packs.