Scenery issues

I posted this same post in FSX Forum but with no luck so I hope for better luck here. At first, my addons are the GEX Europe, PILOT'S Ultimate Europe and Aerosoft Mega Airports series of Zürich and Athens airports, plus the PMDG MD-11. After taking off from Zürich I saw some blue tiles or squares on the land part.There are times that I get those tiles not on the land but on the water! There are also times with no tiles at all and everything works perfect. What causes these tiles to appear? My next problem is after 2 hours of flying and landing in Athens some buildings were transparent! Before the flight, I checked Athens X scenery and all buildings were there with no transparency.

I really don't know what causes the problem and as I said above this is sometimes not always!

I would be so thankfull when somebody shed some light to these strange issues.

Thank you in advance