One thing that cannot be ruled out is valve sealing. If it is erratic, then this will happen. The only easy way to possible see if this is the issue is to try to clean the valves, by using some Rislone or other valve treatment, to clean the intake valves at least. If it works, then good. If it does not work, then that does not completely rule out erratic valve sealing; it could be worn valve guides. Partial disassembly is needed for that: pull a few valve springs and manually check a few guides for wear.
Ring sealing is another possibility.
Since you are working on this, OP. Measure all the resistances of the spark plug wires. They should typically be in the range of 3000 to 6000 ohms and none should be over 10000 ohms. And check spark plugs for tight caps on the upper ends, and firm seating. How new are the plugs?
