Note that with only +0.3 DDR2 voltage you are running your RAM at 2.1V. The rated/required voltage to achieve top speed and tightest timings would be +0.4V ie 1.8V + 0.4V = 2.2V.

Also consider going back to 200MHz true FSB (800MHz rated. This, although not making the problem worse may also not be helping get things stable. If you can get 12.5 x 200 stable first then by all means try 9.5 x 266 as it will give a decent performance boost at this kind of FSB level and with DDR800 RAM.

Setting tRead manually would be a good thing to try too. Run Memset and see if you can tell what the current value is (Called Performance Level in that program). Sometimes when on Auto it can't read the value correctly. If this is the case manually try tRead=6 to start. I've just finished a biuld using a E5400 and some DDR800 value RAM and it was stable at tRead=5.