Whooprsie! Forgot about frame tweening! ;-)
Does that mean you've solved the problem?
If not, reduce your code to the bare minimum number of lines necessary to reproduce the problem (preferably with no loading of media) and post it.
When you say "sprite", do you mean Blitz3D Sprites or images drawn with 2d drawing routines?
On a hunch, try increasing your bit depth to 32 - that "solved" a Z-Buffer problem I was having in fullscreen.