Hi there. I've had similar issues in the past, but they were always related to floating point values. Nothing about your code pops out at me. Are you sure you've replaced all floats to ints? What is your player's movement speed, and what is zoom set to?
depending on the player's position on the map and direction of movement
Do you notice a common trait when this happens? Does it always, for example, happen when the player is moving right, or on tile 52? When exactly does this happen? Also, by "flickering/shimmering" between tiles, do you mean that the clear_to_color is seen between tiles?
Sort of like this? This happened to me when my camera pan speed was 0.5; it resulted in half-pixel misplacement.