kernel_task causing 70% CPU usage???

Hello all,

I'm having problems with my late-2011 15" MBP. After a couple hours of usage the computer becomes very, very slow. I noticed in the Activity Monitor that the kernel_task is using 500% of the CPU (seems weird, but anyway), causing my system CPU usage to bounce to 70%.

In case it's relevant, my MBP battery is 100% dead. We've ordered a new one, but until it comes I'm running on A/C power exclusively.

Any ideas of how to solve? Screenshot is below.

Cheers.

I've also tried resetting the SMC, with no luck. I get about an hour of good usage out of the laptop before it grinds to a halt.