I wonder what colour the exhaust is. If it is sooting up the plugs pretty fast then maybe you have a oil leak on one of the valves and as it idles it builds up on the plugs and after a bit there is too much oil in the cylinder and soot on the plug so it kinda starts to diesel and cause the problem try cleaning out the plugs again start it and try bringing up the rpm shortly after starting it and then leave it there for a min or two and see if it starts to faulter again or if it seems to improve. if the valves are leaking oil down them you would get great compression and not know you have a bad compression ring. I know my 42 idles great but is rough at righer rpm and it has low compression readings.