Buick 225 "dauntless" V6 won't idle, dies under load.

I bought this Jeepster Commando w/ the Buick 225 "dauntless" V6. It is extremely clean and only has 36k original miles. It is bone stock. When I bought it it had trouble idling but once you got it feathered out and the RPMs up it did fine. The jeep had been sitting for years so I figured the carb needed a rebuild and I went ahead and rebuilt the carb (2v rochester). It looked surprisingly clean inside the carb but I went ahead and rebuilt it and put it back on. No change in performance. In fact, it was worse than before. Seemed to be starving for fuel. I replaced the fuel pump. No change. Replaced the fuel tank with a new one and replaced all fuel lines and installed new fuel filters. No change. Now that I replaced the whole fuel system with no change in performance I decided to replace the stock distributer with an HEI. Spent the money and got the CRT HEI which I had heard was the best. Installed with a new set of 8 mm wires. Had to make some mods to the intake and brackets but got it in. Also just decided to go with a professionally rebuilt carb just in case my original carb had some structural issues not fixable with the rebuild. It was just like my old carb, rochester 2v. After putting the new carb on and the HEI it fired up and ran perfect for about 20 miles and then it started having trouble idling again. The more I drove it the worse it has gotten. I've messed with everything I can think of (timing, low idle screws etc.) and replaced everything I think would cause this. I'm stuck. Any help would greatly be appreciated. I'm thinking maybe the timing chain skipped a tooth or something but at times it will run very smooth and then 5 minutes later it craps out. It is not consistent in it's behavior so it makes it even harder to diagnose.