That's often called "pixel corruption" and indicates a bug or error in managing the video RAM.
The first thing that you should try is updating your graphics drivers from the website of your graphics card's manufacturer. Also check that you meet all the requirements for your shaders mod (especially OpenGL version support).
If that doesn't solve it, then there are a couple of possibilities. Your graphics card could be running too hot, or it may be dying (test other games to see if this problem not unique to Minecraft), or your graphics drivers may not be properly configured for your hardware (open the driver software and disable special features), or one of your shaders may have a bug in it (a single bug in a single shader can mess-up all of them).
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My mods: Base Metals, Power Advantage, Steam Advantage, Electric Advantage, Minecraft Mineralogy, Dr. Cyano's Lootable Bodies, Dr. Cyano's Wonderful Wands & Wizarding Robes