Freerk
Being none of the more obvious things have worked, may I suggest a few more radical checks as follows:
1) Remove your crankcase magnetic plug and check it for excess metal, possibly your cam shaft and/or lifters are on their way out?
2) Remove all tappet covers and top spark plugs, with ignition grounded rotate engine and check each cylinder for complete valve movement. Cross check distance of movement of each rocker shaft with the other to identify any that may be short of full valve movement. A bad cam and/or lifter or bent pushrods could be your problem?
3) Check "very carefully" the fuel inlet chamber on both carbs just above the needle & seat. I had on one occasion a small paint flake randomly limit the fuel flow of one carb as it swirled around in the small chamber just above where the brass seat for the float needle is pressed into the carb body housing. It took forever to find this problem even after taking the carbs apart many times and cleaning them. Each time the paint flake would stick to the wall of the chamber eluding us from seeing it! You need to look very closely into this small fuel inlet chamber to spot any such foreign debris like this! We finally figured it out by making a special set of carb float bowels modified with built in windows so we could watch the float bowel fuel level under different load conditions during ground runs. Because it would only randomly block the fuel flow, we had to run it forever before it finally acted up on the ground runs allowing us to spot the troubled carb!
4) With top plugs removed and ignition grounded, rotate engine over by hand checking for tight spots. If you identify a tight spot, you may have a twisted crank. Of course this check can be done the same time you do the Rocker Arm and Valve movement check(#2 above)
Good luck and don't give up, the problem will eventually be discovered and is often more then one thing adding to the difficulty in narrowing down the problem!
