Macbook Pro A1226 CPU running half speed

Hello,

I just have the really weird problem, my MacBook Pro has been run very oddly and crahses a lot, it's been a very good warrior since I purchased in 2007, traveled around the US, Mexico and Canada doing a lot of stuff and being a real trooper in all my projects. Unfortunately I don't know exactly when this issue started, I tried several software fixes(reinstalling from Tiger to OS X Lion) then I run the Apple Hardware Test from its original discs and the AHT reports that everything is fine, but my MBP runs very slowly, from scoring +3000 in geekbench 2 to scoring 817 points! it has been a nightmare, so I managed to get a deeper diagnostic by a local Apple Certified Technician, I learn that one of the temp sensor was reporting higher temp than operating limits. the "GPU 0 Die heatsink temp sensor" to be specific, everything else reported a PASSED score. I have to say that I notice the problem because I use Final Cut Pro and Maya a lot, that means I need my mac to render a lot of things, When I start a CPU intesive task, as rendering is, the fans go all the way up to the highest speeds (6k/rpm) and I can know by that, that my MBP is running at it's limits, but suddenly last week I sent some renders but the fans never accelerated to the highest speed, instead they ran at the 2k/rpm, I teared my MBP apart and give a new thermal paste treatment, clean all the dust, and make sure all was tight and clean, but it has the same problem, OS X reports that my CPU is running at full 2.4ghz, I used HarwdareMonitor, iStat Pro, MacCPU ID and ran Geekbench and Cinebench again, the scores were the same, Cinebench scores 0.25!! , so I scratch my head a little and think a lot, I started to think that there were a problem with the cpu itself, maybe one of the cores just died, so as all the OS X utilites reported that my CPU is running at the 2.4 GHz, I made a partition to install Windows XP so I can install CPU-z, GPU-Z and HWMonitor so I could know if my CPU was running ok, After the installation I ran geekbench under Windows and the CPU-Z reported that my cpu was running at 1192Mhz!!! Even when was at full throttle. So I found the problem, the CPU clock speed was lowered to 6x instead of 12x(200Mhz x 12x=2.4GHz) ! I still don't know if this is caused by the faulty thermal sensor, or if the logic-board is dying. Anyways I did a SMC and NVRAM reset, and that helped a little by increasing the Benchmark from 817 to 1816, still very low from the original +3000 scored a few motnhs a go, but as long as I don't have the new pieces to replace them, I'm not sure what's going on.

Does any of you have the same problem? I really love this MBP, but it has a lot of battles and medals to put it to rest, "she" has some battle scars on her skin: a LogicBoard replaced in 2008 due to the Nvidia 8600m GT faulty chip, RAM upgraded, Hard Drive upgraded, Left I/O replaced, right fan refurbished (BTW I did all those repaiments but the Logicboard replace, that one was made by an Apple Certified Techinician in Salt Lake City)

Please if anyone knows if this has a fix I would appreciate it. May be is time to let it go.

THANKS!