When I started out simming, I was sure my PC had enough grunt to cope, but when I went online flying with the MOG guys, my FPS were never great, and once the action started, my flight became a slide show. My frames would freeze, and when they started again I had often already crashed, or even worse, crashed, re-spawned and crashed again. Sometimes I would wreck half the AI wingmen aircraft which was very frustrating to me and fellow players.
To cut to the chase, the MOG guys were very knowledgeable, and took me through a process of upgrading my PC, tweaking it, and step by step upgrading the hardware. I put in more RAM, I put in a new video card, I tested the cooling, I surveyed the wattage use, and even upgraded my power unit. All of this was a superb education, but didn't nail the problem of my framerates dropping away. (A good point I need to add is trusting your labels - Just because a power unit says it cranks out 300W, doesn't mean it actually does).
On paper, my CPU was 1.2 ghz or something, I forget now, - but on paper that should have been enough. My CPU was a 'Celeron' chip, and therein lay the problem. The Celeron chip (now correct me if I'm wrong) was never intended to be a gaming chip, and it's performance catered for business use. On paper it was fine, but CFS3 was taking it to the limit.
Eventually I was cured. - New PC, which actually did a lot more, but cost less than the old one.