Hi Machead -
Have you run (or have access to) the Apple Service Diagnostics 3S108 disc or EFI boot? I was having a similar issue with 2 different A1150's...
The ASD software pointed out a failing sensor (not every time, intermittently) on one MBP CD, and a failing GPU on the other. For whatever reason (never determined precise cause), the 2nd model's graphics chip kept getting really hot (even with either the SMC Fan Control App or the Fan Control Preferencepane). If you read up on the Core Duo generations' CPU and GPU chipsets, they are known to overheat (the industry measured percentage is in the high 30's). I had the same GPU overheat issues with an iMac of the same generation of chips: (T2400, T2500, T2600) & (ATI & ATI Mobility - Radeon X1600).
My solution for the first was to replace both sensors with known working sensors... worked great. Pretty straight forward repair.
My solution for the second took a bit more effort. I did this procedure twice... the solution "took" after the second attempt and it runs fine now. Essentially, without going into the complete disassembly procedure... I removed EVERYTHING (cables, sensors, pcb's - logic & DC-in, you name it). Cleaned everything thoroughly using canned air (fans & case), q-tips, electronics anti-static brush and high grade ethanol. For you, the key I believe is the fans, the venting, the heatsink, areas under the heatsink and fans. Those are the parts you really have to actually remove and clean (do not use anything but quality ethanol on circuit board or chipsets, canned air on the fans and maybe q-tip if you want to be extra thorough). I just removed everything to be thorough and give it a solid cleaning. Heat comes from multiple sites in the laptop, and reducing the overall heat was my goal.
The followup to the good cleaning was using a high-quality high-heat tolerant thermal paste. The cheap stuff is a blend of ceramic and silicon (great for some things - not for this repair, though)... the more expensive blends are polysynthetic silver (a few other types too). I personally find the Artic Silver 5 to be the most tolerant of high heat in tight spaces, but USE SPARINGLY. One dot (perhaps 2-3mm diameter) of perhaps (guessing) .15 cc's per heatsink pad.
To recap... I cleaned components listed and replaced the thermal paste twice. The first time improved lagging performance (from heat) and the frequency of shutdowns, but the problem didn't go away. The second time solved the problem. I still run the Fan Control PreferencePane as a safe measure (bumped up base speed of 1000rpm to 1500pm and kept temp tolerance range at 40-70).
Questions? Need ASD?