I asked because it turned out I had a leak that drew from below the valley pan. The plugs had an oily appearance. If you don't have that, I think you can rule out this possibility. You still might have something on the manifold side of the valley pan gasket, but let's assume you don't for now since you think these symptoms might have been there all along.
I do not think a leak would necessarily cause the idle to cycle back and forth. For this I would look to the ignition. Someone else may have a different idea.
I also asked about the MSD and coil because I had an issue with the coil (and possibly MSD) that caused more of a rhythmic idling quality (up and down) after the engine heated up. That was solved with a new coil and I also swapped the MSD at the time. The problem went away at that point.
I asked about the ignition cutting out because I had a situation where I had an intermittent miss that was related to power making it to the MSD. This was also seen in the tach. I saw this in mild acceleration and it only happened when hot. It was the connection to the fusible link that was loose.
You may know more than me and as you can see I have made lots of problems for myself, but generally have solved them. I would be careful about spraying brake cleaner into the intake because some brake cleaners combust to form bad products. I spent a lot of time trying to figure out things with an A/F gauge and it will help. I suspect the bouncing around is not directly linked to a vacuum leak because the leak should not necessarily bounce around, but be a constant addition of fuel free air. The idle will run well rich for a wile and then foul plugs so that is not the solution.
I would check out the coil using an ohm meter (I had to look online to do it). If it comes out at spec, then I think you are OK, but it is not out of the realm of possibility that the coil could do something different when hot. I would also check all connections between the key and the coil to rule out something that was intermittently cutting out, but I think you would see that in the tach. Assuming it is not the MSD (if you are flush, you could buy (or borrow) another to try, but for now assume it is OK).
It sounds like your main metering circuit (jets) are good so I would leave those. I think you have done this, but I would disconnect the vacuum advance and set the distributor to the idle setting that gave the highest RPM. I would then curve the distributor to make sure the max advance was not too high. Then I would use a vacuum gauge to set the idle mixture screws for the carburetor (I think you have done this already too). Then I would make the idle mixture screw setting a little rich (maybe 1/4 turn more open). After that I would take some Air fuel readings. They should be pretty stable at a any given RPM. If you are making traces (logging), you will see some oscillations. I wouldn't worry too much about those. They will go up and down by a lot if you are on or off the gas, but if the pedal is steady, I would assume they should sit +/- 0.5 to 1 unit of A/F. I would take A/F readings at 1500, 2000, 2500 and 3000 with a very steady pedal and see where it sits. If you are at 15 -16, you might be too lean and this might give the surging.
I have written way too much to digest and apologize.
