after inyectors check wiring harness, i got that problem,
i asume was inyector, but finally where thas wires, all burned, ALL

Thanks all.

Yes it is W140 body (but year 1991.. they started one year earlier in
Europe).

I waited until the morning. All plugs seemed dry and all with the same
(little) amount of coating. I tried to see on piston tops too, and
they were also dry. I tried to wait an hour while frequently spinning
the fuel pumps (but without starting). Still all dry.

That would indicate, that injectors are OK. Or are there some other
means to check it deeper?

Dr. Chino suggest wiring harness. Yes, they were bad some 2 years ago,
and I replaced the bad sections of the wires (soldered and insulated
with heat shrink tubes) at that time. Still there may be something,
and I may have to look in that direction again.

When i starts (when problem is present), it chokes somewhat in the
first 5-10 seconds or so, as if it has to get rid of the gas vapour,
and then it runs fine. Also it starts well when stopped for only few
hours. The problem occurs only when it has been stopped for several
hours while sun shines on the hood.

So I lean more towards a fuel system problem.

To my knowledge, once the fuel pumps have build up pressure, there is
only 3 ways, the pressure can escape: Output valves of the pumps (have
been replaced), pressure regulator (has been replaced) and injectors.
Plus, of course, leakage in the piping (no visible signs and no
smell). Am I missing something here?