I believe you're having Radeon Crimson driver issues. Check your core temps on your GPU. If you're running a factory overclocked card, there's been issues with the drivers in that they turn the fan speen to manual on low and when the card runs something intensive, the temp goes all the way to 90 degrees celsius and it starts to thermal throttle or shutdown.
Its important that you do the following:
Uninstall all the AMD drivers completely (Yes completely, don't just update). Reboot. Install the latest (v16). Reboot again. This should fix your issues.
Also, Install MSI Afterburner (it works on all GPUs, as its based on
Rivatuner) and turn GPU temp logging on to keep those temps in check.
A quick test is that you should hear your GPU fan spin up when you start mining.
Please be aware that running a GPU 24/7 will cause the fan to eventually fail. (They will fail within months as they're not made to run at max speed 24/7. You should perhaps look at getting your GPU watercooled, or even mineral oil submerged if it makes financial sense.