You're right. Sorry for holding off info, but I'm soo deep in this that I'm on the verge of insanity, so here it goes...

My Euro '78 280SLC has this weird problem: The car runs great, as long as I don't exceed about 4000 RPM at full throttle. If I do, the car start to miss fire on one, and sometimes two cylinders at idle and up to about 4000 RPM. If I then proceed to turn it off for about 30 minutes or so it start right up and runs fine again.
Also, there is an accumulation of oil inside the intake manifold in the form of a 2mm deep puddle. I have no explanation for where it is coming from, as there is no blowby (the breather on the valve cover actually has a little suction).
Other symptoms include:
-Slight oil fouling of the plugs, but no blue smoke or blowby;
-Manifold vacuum pressure at idle of only 14inHg (no EGR or other smog devices);

Things I have done so far (in no particular order);
-Adjusted valve gap (0,10mm intake / 0,25mm exhaust);
-Changed spark plugs (NGK BP6ES gaped to 0,8mm);
-Changed leads, rotor and dist. cap;
-Replaced the coil;
-Adjusted ignition as per workshop manual;
-Checked injection system for correct pressure, and eliminated both the fuel distributor and the WUR;
-Played with the mixture screw more times than I can remember...
-Checked for vacuum leaks (found none);

I just did a dry and wet compression test with the following results (valves were adjusted only 3 days ago):

Cyl / dry / wet / difference
1 - 120 / 130 / 10
2 - 118 / 135 / 17
3 - 120 / 132 / 12
4 - 122 / 135 / 13
5 - 122 / 135 / 13
6 - 122 / 135 / 13

If I have ring or cylinder issues, shouldn't the difference between wet and dry be a lot higher then just the 13 psi average that I've got? Or am I looking at an engine rebuild?