I've actually done this. This is a common problem for PBG4s. The fix is to replace the whole logic board. I got one for about $175 off of eBay. I found the fix to be relatively easy, though I'm pretty good with these things.
Oddly enough, I've actually got a 1.67 GHz board for a 15" PBG4 that I've been meaning to put up on eBay but haven't gotten around to doing it yet... I'd be willing to sell it to you, PM me if you're interested.