If your GPU useage jumps with the frame drops, your CPU will be momenteraly bottlenecked by the GPU and not work as hard.
If your FPS are capped at 60, your GPU may not have to work hard enopugh to boost all the way up. If you really think your CPU is problematic, runs a bechmeark, like inter extream tuning utility, as see what the frequency does then, when GPU is out of the equation.