Trees become ghosty

Dear All,

I am running a Billboard application.When I am moving(using keyboard)very close to the viewpoint,trees turns ghosty(trees nmorphs to a rectangle).Also it turns ghosty when I move with left and right keys.I am using spherical billboarding and cyclindrical billboarding.I am using WIN2000,P3 with Matrox Graphics accelerator card.

What may be problem.

With regards