I think there is something wrong with my Sapphire Radeon R9 390 Nitro graphics card. When I play games, core clock is around 700-900 MHz and almost never reaches it's maximum clock of 1040 MHz or my overclock of 1120 MHz. Because of this the games I play (like Watch_dogs or Assassin's Creed Unity/Syndicate, ...) don't look smooth even at 60+ FPS and it is making the games stutter everytime the clock drops. I have already tried running the card in another PC (with i3-3100 and 12 GB RAM) and the drops were even bigger (around 600 MHz all the time) and I also had to lower the details. My PC specs: CPU: AMD FX-8320, RAM: Kingston HyperX Beast 8 GB 1866 MHz, MOBO: MSI 970 Gaming, PSU: Corsair RM750 and the overclocking program is MSI Afterburner and other values in it: core voltage: +100 mV, aux voltage: +100 mV, memory clock: 1700 MHz (is holds this clock fine with no drops at all).
What could cause this? Thank you very much for answers!