Kernel_task at %300 ! Nothing seems to work

Question

Hi
A few months ago my computer gradually started to run slow. At first I didn't really notice it but now it is almost unusable.
I'm running Mountain Lion OS X 10.8.2 on a late 2011 MBP, Processor: 2.8 GHz Intel Core i7, Memory 4GB 1333 MHz DDR3
The problem describes itself like this:
The computer is really slow and lagging allready at the gray apple logo screen at startup, doesn't matter if it's cold or warm before or if it's a restart or a normal start. After (trying) to use t for 15-20 min it sometimes gets allright. If it is working ok and the monitor shuts down it starts lagging again. I've been using the caffeine application to prevent it shutting down durin class, and i've noticed if I leave it on without touching anything it usally gets better after 10 min.
After browsing a bit I thought the Kernel_task is the problem (%CPU is 300+)
I followed everything here https://discussions.apple.com/thread/4405099?start=15&tstart=0
except http://www.rdoxenham.com/?p=259 (I find it a bit intimidating :$)
I got an error after the hardware check: 4SNS/1/40000001: ... (can't remember the last letters)
Yesterday, thinking the problem was a 3rd party software and not knowing which one, I uninstalled everything. It seemed to work. The Kernel_task went down to just 3 %CPU and the computer worked like it was new out of the store.
Like apple advices, I put it to slep last night instead of turning it off. This morning when waking it up it was like yesterday never happened. The computer is lagging again and Kernel_task is again up to over 300 %. I've been watching the activity monitor while using it today, and during the good times (those few minutes it actually works although not as well as yesterday) the Kernel_task is still over 300%
The activity monitor shows now:
%CPU user  <5%
%CPU system 85 - 90% !
     Kernel_task %CPU 300-350% , Real Mem ca 300 - 350MB
     All other processes <5%
System Memory:
     Free: 2.23 GB
     Wired: 843 MB
     Active: 820 MB
     Inactive: 143.4 MB
     VM size: ca 200 GB
     Page ins: 0-3 kb/sec
     Page outs: 0 bytes/sec
     Swap used: 0 bytes
Disk Usage:
     Utilized: 300 GB
     Free: 450 GB

Answer 1

If you got a hardware test error code, then there is really not much we can do. Take it in for service.
You can run through this list of software fixes, but the hardware issue might cause more after you fix things.
..Step by Step to fix your Mac