   Lost zbuffer on lower resolutions
  I, im working on an application and i was developing it on a 1024x768 resolution (but i intended to use any resolution that the card support), anyways, recently i changed the resolution to lowers ones like 320x240 or 640x480, and the zbuffer seems to be screwed, all objects draws over eachothers, something that dont happen on 800x600 or moreIm missing something ? some State ?   
  Make sure that your near and far clip planes are good such as 1.0f and 1000.0f. What is your depth/stencil buffer set at? Have you tried an AutoDepthStencil? Something like this...presentParams.AutoDepthStencilFormat = DepthFormat.D24S8; //24 bits for the depth and 8 bits for the stencilpresentParams.EnableAutoDepthStencil = true; //Let direct3d handle the depth buffers for the application//Then create your device...I hope this helps.Take care.   ;  I actually have that configuration on my device   