Does it happen all the time or just when heavy loads like playing a game?
If it only happens during game playing I would check the GPU's Hot Spot that it isn't going over 110c. otherwise it could be any of the below answers or more that other Users can come up with.
If happens intermittently all the time I will still check the temperature but most likely it can be a driver or physical hardware issue (loose or defective Video cables, GPU not probably latched to the PCIe slot, AMD driver, possibly configuration issue).