Help Needed - Rough Hot Idle - 1989 420 SEL

I have spent numerous hours reading many postings here related to rough idle and it seems I am in for the big ticket fuel distributor. Please give me your opinion before I replace it.
The car is a 1989 420 SEL (W126) , 221K KM. I have a rough idle, both warm and cold, sometimes. It seems one, possibly 2, cylinders are missing, sometimes. It will idle for 20 secs. perfectly and then it will start to miss-fire, and the engine rocks, missing for 30 secs, and then run perfectly again, the cycles are not consistent, sometime 45/15 sec. and other periods. At non-idle the car does not seem to miss fire.
It has been scope tested, there seems to be no electrical problems. The dist. cap was opened, the cap was clean, no arcing or deposits. Pulled wire to cyl. #8, tip was loose, and was replaced. Replaced the injectors and holders, flushed the injectors before installation. Gas line from the tank has been replaced, gas filter has been replaced, filter at the fuel distributor has been replaced, Plugs have only 4k KM, and are the Bosch copper, W8DC. O2 sensor replaced 2K KM ago. A compression test showed all cylinders 180 PSI, except #8, which was 140 PSI. Performed vacuum and air leak test, no leaks detected. Both cams replaced, all rockers replaced. Chain replaced 4K KM ago. Checked both cam timing marks, on the shaft compared to the line on the side of the engine, both left and right sides lined-up, the timing is 0 degrees.
While idling hot, opened cylinder #8 fuel injector line at the fuel distributor, no change in engine performance. Took the line off the distributor and no fuel bubbling out. Tried the same thing for cylinder #4 injector line, engine starting rocking more, a noticable difference. So it seems cyl. #8 is not getting fuel at idle. Loosened #1 cyl. injector line and the fuel looked dark but then cleared-up, but did affect engine performance too. I am trying a bottle or two fuel system cleaner.
Does the fuel distributor need to be changed? What should I check or do next?
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I replaced the fuel distributor with a rebuilt one but rough idle is still exists intermitently. Did a leakdown test on #8 and it showed 40 psi and #5 showed 10 psi. Do I need to pull the head? If it was valves or rings, I would think it would be a consistent rough idle. Comments?

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