Help - Black flickering screen

Hi, I am using the PMDG 737 expansion, along with the PMDG 800/900 series expansion. I am experiencing a drop out of textures (areas of the screen go black)but then return to normal after a second or so, during all phases of flight, from taxi to landing. It is highly frustrating, as my system does not do this on any other add-on aircraft. Does anyone have any suggestions; help would be appreciated. I wonder if it could be a clash with Gary Summons scenery or VFR scenery. In all other ways, my setup runs FSCOF with high Frame rates and I am more than happy with performance.For the record:2.4 P4 (O/C to 2.6)512 DDRGF6800 128mbWin XP HomeUk2000 scenery (Parts 1-6 & Heathrow pro)VFR UK scenery (all)Cheers Andy:-bang