kernel_task hogging CPU

Mid-2010 MBP, Core 2 Duo 2.4Ghz, 8gb RAM, shipped with Snow Leopard, now running Lion with latest updates.

For about 2 weeks, kernel_task has been hogging my CPU. Performing almost any task makes it shoot up to 50-90% (or 100-180% counting cores separately), with other processes taking up the remainder, causing the machine to hang for a long time. It happens when I do almost anything - copying files to a thumb drive, opening an application, opening some tabs in a browser, playing an AVI or MKV file, etc. Sometimes it will perform one task OK, esp if all other apps are closed, but eg playing a movie while a couple browsers are open is almost always impossible. The actual app process doesn't seem to take more than usual of CPU, and when doing nothing kernel_task idles along at 1-5% (even with lots of apps open), but almost any action with any app causes the kernel to go crazy.

It seems slow to boot up as well.

There is no obvious cause but it happened shortly after upgrading to Dragon Dictate 2.5.2 from 2.5.1. Also around that time (I think after), the fan was making loud, unhealthy noises so I opened the back and cleaned the fluff out of it. Don't think I touched anything else. Fan still doesn't sound quite right but the temperature is ok.

I have tried the following:
- uninstall all versions of DDictate, incl preference files
- remove all startup apps (dropbox, igetter, etc)
- clean install of OS
- repair disk permissions (found some errors in javascript)
- disk repair (using install disk)
- verify disk (after booting)
- file system check: /sbin/fschck -f in single user mode
- reset SMC (no reason it should help, but after the disk check above when booting it said something like "SMC InitialHelper Error" and another SMC error.)
- Rember (ram check)
- TechTool Pro (ram, hard disk, video memory)
- reset PRAM and NVRAM
- check the temp: CPU is the same as before (65-75C); the highest, according to iStatMenus, is Northbridge 2 at up to 80 (normally more like 70)
- virus scan with Avast (installed it specially for this, then had to disable as ate up CPU)
- ensure plenty of free space on hard disk (currently 100gb)

None of this seemed to find or solve the problem.

I can't do the Apple Hardware Test as I don't have the original 10.6 disks with me, and my model doesn't support the EFI upgrade method.

It is out of warranty, and I am not near a Mac service centre.

Suggestions will be much appreciated.

It seems it was a fan issue after all. I installed SMC Fan Control and realised it was only getting up to about 1100 rpm even when I set the minimum at 5000. I took the fan out, took it apart, cleaned it again, oiled the spindle, and replaced it carefully. It's now running at about 5000 rpm and there are no more problems with kernel_task - I'm currently playing 2 movies, running 2 browsers, downloading several files, backing up to Time Machine and processing images in Photoshop.

I'm quite surprised because the weather is hot where I live so CPU temperatures are normally around 70° (often 80°) according to iStat Menus, even when the fans are working properly, and it didn't seem to affect performance until now.

Running the fan at that speed using SMCFan Control will eventually destroy the fan possibly creating overheating problems for your Mac and fan replacement.

We recommend allowing your Mac's temperature sensors to control the fan speeds just as Apple has designed them to, not SMCFan Control or any other app.