Yesterday I was flying the same route again, just to try to end this flight, and once more I had this frame drop after 01:30 to 02:00 hours. I use this high "Cloud Area Covered" setting to to take use of the features the Real Weather Connector brings at all.
Someone of another X-Plane forum wrote me, that he knows the problem and all I should do is simply open the SkyMaxx Confirguration and change anything, so that the clouds will reload. This can't be a solution forever, but I did that, though I used the button, that's below to reset the whole configuration as Frank from Sundog Software suggested - that helped me getting fps back immediately.
I will repeat this flight today but this time with standard configuration from the beginning. I will load the weather condition from yesterday using FS Global Real Weather to stay comparable.
thanks for trying to find the reason for this behavior
Carsten