If you don't know, likely you are using the one that came with your AMD CPU. Those tend to provide minimal levels of performance over time. Adding a stronger GPU can put more of a load on your CPU and generate more heat. You could also have a lot of dust built-up in the CPU heatsink that can be removed with some compressed air.
BTW, are you overclocking either the CPU or the GPU? If so, stop and see if the drop continues.