Hash rate goes down after a few minutes on my GPU

I recently bought a AMD 7000 series GPU. When I reboot the machine the card hashes at about 250 hash/s … however after a few minutes it goes down to 50 Hash/sec … is there anything I can do?
I am using Claymore miner, 4GB RAM, 1200 watt eveo PSU.

Try OpenClMiner MEGA

Run eth_pp_ua-ocl.bat to start mining.

Claymore is tweaked only for certain GPUs which could be the problem.