Had the same problem with my 260E, but even more weird; that is, in freezing cold (-16 degrees Celsius) it started at the first turn of the key and ran well, but a few days after, when the weather had warmed up, it refused to start.
After a half mile of towing (could not risk to damage the starter motor by cranking endlessly) it started, but huffed, puffed, vibrated like a Diesel. When it finally had warmed up, it ran well (but with weak acceleration).
The story has repeated for two more times(long cranking, hard start), but not in consecutive days. After that, the car went back to normal by itself, without any intervention, and now runs well & safe, but it still doesn't accelerate as quick as it should.
Suspected it had to have been either the injectors(never cleaned) or the cold start injector valve, since the gasoline here is always dirty(more accurate would be to say it's "dirt with some gasoline in it"
)
~Nautilus