Ehh at below 1.5v on water you should be just fine. I doubt its degregration. Ive ran dual core 6600 on phase at 1.65 for 24/7 for a long time and it ran before me at around 1.5v on water (person I bought it from) and now its on air at 3.1ghz 1.32v and doing jsut fine folding 24/7 and whatever else gaming you name it.
There could be a few different things at play that you gotta look at. I would try a lower multi and see how high you can get it stable. You may have to worry about other voltages other then you cpu and ram. YOur vmch is gonna have to be increased for quads at high fsb. Even your icho and such may need to be increased to handle the higher fsb even though you would think this wouldnt be necessary.
Try running 7 mutli and see how high it goes. Some times a 6 multi can be unstable for some reason on certain boards.
Also play with your reference voltages.
Some old threads on these voltages but it may get you 5 or 10 fsb extra.
Also fsb holes SHOULD be elimintated for the most part with P35 and later boards. It was present in 680 and earlier boards very badly and also to a certain degree in 965 boards due to the strap changes. I havent encountered one on my IP35PRO but eh.
Good luck, and you never know it may be a fsb wall or the limit of the ocing for that cpu. Or it could be your ram or some other feature.