Computers degrade over time, it's inevitable. For example a 4.0 GHz new computer may slow all the way down to 2.0 GHz over a couple of years.
Your comp may have simply hit that time in its lifetime where its speed drops, causing a massive FPS downfall. The only thing I can think is to just get a new system, continuing to try to fix it with new cards and processors simply isn't worth it. It seems to have nothing to do with CoD engine, other than the engine itself takes up too much GPU and RAM for your system, causing it to lag.