Hi! I agree with 3dRaven. Years ago a customer wrote me a message with a verry similar probleme. My solution was to reduce the number of textures in the material. So i needed a rust and a grounge texture in that material(similar shape, different color), and I removed the grounge texture, modified the color of the rust texture with hsl node, and reused it as grounge. So the overal number of textures was reduced, and after that the material worked flawlesly. Sorry for my english, I am learning yet:D