If you have the same problem using win7 and win10,then probably your GPU is faulty.
You can try some old drivers. 16.12 and 17.1 are very problematic
1.Delete C:\AMD folder
Thank you,I'll try it.
You should check GPU/VRM temps and clocks (preferably with app with OSD like afterburner with RTSS for example) or having GPU-z running in background. If GPU-z shows you power, I would definitively leave it in background to check power consumption when fps spikes occurs too.
As for drivers, 16.7.2 are last completely functional drivers for 1st gen GCN cards. With newer drivers, your card will became faulty since "UVD clocks" bug does not exists.