Intermittent problems are always the toughest to solve. when you say the engine sounds rich when the problem occurs that may very well be but by what you are describing about the mixture screws not having much effect on idle quality that is a lean condition in the Carburetors idle circuit or a vacuum leak. This may or may not have anything to do with your intermittent problem and if it doesn't still needs to be addressed. Have you meticulously checked for vacuum leaks? Another thing to look at is the idle stop solenoid if you are using the factory setup. If dropping out intermittently it could be the cause. One last thing to consider is spark. If you have a cylinder dropping out or partially dropping out due to crossfire intermittently. Check over your system carefully, things like overlapping secondary wires, wires touching each other and wires touching metal. the easiest way to find light crossfires or groundings is in the dark. Also don't forget to look at the plugs, and especially look closely at the cap for any carbon tracking between posts. all that said with the limited amount of info about this problem My guess would be carburetion and checking for Vacuum leaks. Just check all the basics and things I talked about before disassembly of your Carburetor. By the way here is a read on deciphering vacuum gauge readings https://www.tpocr.com/Maniflod_Vacuum.html