I'm having the exact same issue as you gents: Samsung 4K monitor & crossfired R9 290s. Mine are XFX double dissipation cards.
Tried the 14.4 & 14.6 drivers, new power supply (silverstone 1000W), each card by itself, new DP cable, no help. Some times it happens just in Windows, most of the time it's in 3D games. It's so frustrating as when it's not happening the games are silky smooth & beautiful!
Tried cleaning & reinstalling the drivers, a fresh install of Windows 8.1, no help. I don't know if 2 cards can both be faulty or whether I'll just have to try a different card all together!