No idea if this will solve it but it looks like a weird problem i had before that made me scratch my head for days except it was happening on a depth buffer. Looks like a sampler state problem.
Try setting the sampler states AddressU and AddressV to Border.
public static SamplerState SS_PointBorder = new SamplerState() { Filter = TextureFilter.Point, FilterMode = TextureFilterMode.Comparison, AddressU = TextureAddressMode.Border, AddressV = TextureAddressMode.Border };