cod 4 only uses 2 threads and hardly uses the second. thats where your cpu falls fat for gaming. your fx has very poor single threaded performance and will bottleneck any gfx card stronger than a gtx 250 when only a single or dual thread is used... sorry you just bought the wrong hardware for gaming im afraid.
your best bet is to to 4.5ghz and hope for the best.
because there more stressful on the system they will use the available resources efficiently. because older games arnt as demanding they will often not push the gfx which means the card is able to produce huge amounts of frames. so many that the cpu cant cope and basically overloads forcing it to tell the gpu to stop till it can catch up. this is where you see the fps drop to near 0.
so you best stick to demanding games. you will get better overall performance.
Actually your problem was an out of Date DX 9. Just because you have the most recent DX 10/11, you still need the most recent DX9 files as well, especially for older games still running in DX9 mode.
Hope this helps!