iMac slowing down?

Okay, there's something unusual happening, I noticed it before but figured it just to be corrupted data.

My iMac seems to be dragging it's heels when it comes to games now, and it's more than merely 'noticeable'. Where before I could play CoD4 on this things at full resolution and maxed graphics, the game is now completely unplayable and choppy, maybe achieving 3 frames a second.

At minimum graphics and resolution.

I deleted the old file and reinstalled, and it is the same - the game is completely unplayable, and other games like CoD5 and Prince of Persia are showing the exact same issues.

I'm running a 3.06GHz with 4GB plugged in, with an NVIDIA 8800GS. The computer was top-end last year and I repeat; it ran CoD4 like a dream but now the game is totally unplayable. This thing is only a year and a month old...

Have others suffered a similar issue, and is there any cure?

Cheers.

Have a hunt thru this and see if u can sort it - http://www.mac-forums.com/forums/os-x-operating-system/93819-basic-os-x-mac-troubleshooting.html - unlikely to be a hardware issue

I suggest you try downloading the free fixit all

AppleJack Project Page

It will clean, empty and help your machine. It is quite possible the whole HD is fragmented and may need serious help. How about a Time Machine backup then a reinstall of Leopard with a format of the HD? Have you used Disk Utility to verify the disk?

I did just run the disk utility and it did assist the game, made it playable but still far below what it once was.

Never seen that troubleshooting bit before, I will definitely go through it, then eye up that Applejack bit. Thanks kindly, I will report findings.