DirectDraw Flip Flicker

I''m developing a Managed DirectX 9 Game in C#. It''s a simple 2D Nibbles (aka Worms/Snakes) implementation. I''m using the standard surface.flip() call to interchange the primary and offscreen buffers. Unfortunately, I''m getting intermittent but non-periodic flicker. When I replace the surface.flip() call with a surface.DrawFast() call the flicker disappears , but (as you would expect) I get a performance hit. Has anyone else had problems with surface.flip resulting in flicker? I was convinced it was my code until I found other samples that suffered from the same flickering while using surface.flip. Thanks for your help.