My only thought is that you need a new pc if you have a budget for it but even then fps can still be an issue regardless of what you fly at the bigger airports. I tend to stick to regional payware airports as the fps tends to be better.

I have an i5 3570K overclocked to 4.5ghz and a 470GTX and at the big airports like EGLL my fps can take a real dive, below what im happy with which is 24fps.

I tend to fly the CS757 ( love that bird best in the sim even with its bugs ) and another developers a320

If your bios is locked and it prevents you from overclocking then a new system would be the only way, depending on where you are in the world it can be cheap to do depending on what you can can use from your current pc.