Any thoughts why my hashrate is dropping?
Either HBCC is turning off somehow (some other program or task starting?) or your fans aren't high enough (put em around at least 3000) to keep the HBM mem cool enough to sustain the high hashrate
I've had the same problem - I don't think it's HBCC, becasue I can restart the miner without touching HBCC and speeds are back to normal. That made me think it was temp - but even with fans over 4000rpm in a cool room it would still happen.
I tried both using the iGPU, as others have suggested, and going into Control Panel - System - Advanced System Settings - Performance Settings, and then switching to adjust for best performance (Disabling all the windows animations). That worked for me.