Bad Axe won't post with 4 gigs of RAM

My Intel 975xbx (rev. 304) will accept three 1 GB sticks of Corasair XMS memory, but when I put a fourth stick in, I get the three beeps and it won't post. I have tried all four sticks in different combinations, so it isn't a particular module that is "bad". I followed the voltage settings recommended by Corsair and have tried altering them, but I can't get all four to work at the same time. They were all purchased together, by the way. I can't find anything on Intel's site or Corsair's sited about a known problem. Any help would be appreciated.

Thanks,

Bill

Core 2 Extreme Processor 2.93
MSI 7950 gxt Video card
Vista Ultimate 64 bit

Yes.

Did you have your RAM timings set manually before the upgrade? 1T? 2T?
Is it set for SPD (Auto detect)?
I would try and relax the timings a little with 2 sticks in and see if it works when you add the 4 back in...

The memory is Corsair XMS DDR2 675 (C5400) with timings of 4, 4, 4, 12. The mobo (when set to automatic) runs it at 5, 5, 5, 15.

I have tried running at 533 (three beeps, no post), 800 (three beeps, no post), and 667 (no beeps, but no post).

I have also tried voltages of 1.9, 2.1, 2.2 and automatic. At 1.9 and 1.8 I get three beeps, no post.

I'm open to suggestion.

Thanks for all your replies so far.

Bill