Weird texture filtering when rendering high resolution

[Fredster]

I have 2 examples attached, both with 20 passes

First image is 1333x750px. Behaves exactly like it's supposed to.
Second image is 2500x1406px. Strange texture filtering is happening in all render channels "diffuse, reflect, glossiness etc etc."

Blur filtering is the standard "1.0" on all textures.
I do have GI disabled, but i've tested it with it on and off, same result

Does anybody know what causes this?

- Thanks!