MBP2010 kernel_task CPU hog: how to debug/fix

Help needed please

My MBP Mid-2010 (A1278) intermittently has a kernel_task attack where the CPU usage jumps to near (or occasionally more than) 200% and the fan spins at 6K+ RPM. This of course means the computer becomes very slow and hard to use.

This machine was given to me because of this problem, and the previous owner 'thumped it' out of frustration and managed to break the glass covering the screen and corrupt the H/D. I've replaced both. When I replaced the glass, I have a feeling I dislodged the connectors to the built-in cam and blue-tooth as they are no longer working. But that doesn't concern me from a feature point of view.

Some examples of the problem:
A few days ago, the machine ran fine for 3 days straight.
At other times, the kernel_task panic occurs randomly after I've logged in.
Quite often, the problem occurs while booting, before the login screen shows.
As I'm writing this post on my Windows m/c, the MBP is working fine, but earlier in the day it was throttling while I was showing my lecturer an assignment I'm working on.

The first thing I did was to run AHT, which reported "No trouble found". I also updated the EFI, and did a hard SMC reset (including the one with the battery unplugged) etc. I booted into safe mode, and tried all the other boot options I found listed on Apple's site. None of these things helped.

Since the fan was spinning loud, I initially thought it was a heat related issue. So I disassembled the logic board and applied a new coat of thermal paste to the CPU and GPU. This unfortunately didn't help. I then installed 'Macs Fan Control' to see the temperatures, and they all seem quite normal. (I posted in another thread here on the basis that my problem was related to the MBP thermal sensors, and included all the specifications of my machine there.)

When I replaced the broken hard disk with a Samsung 840 EVO SSD, I installed OSX 10.6 from the DVD and proceeded to upgrade through to 10.9 via the Apple Store. The problem was present all the way through the upgrading process, and I would simply wait till the machine settled before proceeding.

What is frustrating about this problem is that it is intermittent and unpredictable, and I believe it is a software issue.

I'm from the Windows world and don't know all the tools available on OSX to diagnose the problem.
I understand that kernel_task is similar to Windows svchost.exe.

1) What kind of information would be useful to look at to try and figure it out. I've looked at the log files using the "Console" app but I'm not really sure what to look for.
2) What tools can be used to diagnose/see what kernel_task is doing?

Cheers,
Nap