The behavior you describe is strange, it might be caused by cool n' quiet (or whatever speedstep is called with AMD). Maybe at 60fps the processor doesn't take enough load so it lowers its' speed assuming nothing CPU demanding is running (since PCSX2 only uses 2/4 cores). But when you set it to 70fps, it causes extra processor load so cool n' quiet never triggers and your speed is consistent. Since your problems are gone, I guess this thread is done.