Flashing Triangles

My system: AMD Athlon 1.5 GHZ, 512 Mb Ram; Ti4200 Card w/ 128 mb..When I run MSFS2004 I get these flashing triangles all over..They dont start to show up until I start to move down the taxiway..Everything else displays properly except for these flashing triangles. Anyone know what this might be or how I can fix the situation?? Thanks