|Did you add any new hardware recently? Broken or incompatible hardware can cause this.|
For example, when my original SoundBlaster X-Fi croaked, I replaced it with an Auzentech X-Fi. This evil card constantly stole 10-12% of my CPU's time (hardware interrupts). It really slowed things down--1080p video playback was nearly impossible, even though it would normally only use 40% of the CPU.
I finally got rid of the Auzentech and put in a Creative X-Fi Titanium (the PCI-E version of the card, which fixes the X-Fi/NF4-SLI issue). The result: No more CPU cycles being wasted on interrupts, and performance shot through the roof.
-So if you've added any hardware, make sure that it's not sharing IRQs with any other hardware.
-If a lot of IRQ sharing is going on, go into your BIOS setup and disable as much onboard fluff as possible (FireWire, secondary GigE, game ports, unused IDE controllers, etc).
-I've read that USB controllers can be particularly nasty in this regard. Unplug all of your USB devices and see if this helps. If it does, try to dedicate each USB controller to one or two devices.
Pentium M oc'ed to 2.82GHz, 667FSB, 2MB L2
3GB of DDR-800 @ 667MHz
Modified PowerMac G4 'Quicksilver' case
HDMI to homebuilt 1080p projector