Overclock worked before, won't now. PLEASE HELP.

Okay, this one is tricky, and I wish I could provide more information, but I honestly don't remember what I did different, if anything at all:

What would cause one to be able to overclock his Northbridge to 2800 previously, but not be able to overclock it past stock later on?

I ran my 955 Black @ 4ghz and OC'd the NB to 2800 mhz for about a year without even a hiccup. Then I updated my BIOS to version F11 from F5, and changed my CPU cooler to a better cooler (Corsair H60 from Transformer 4) and now I can't even overclock it to 2200mhz). I reflashed my bios to F5 again, but no go.

Is it possible that I screwed up my CPU somehow? Is it possible that the new flash isn't being completely overwritten with the old?

Is there anyone out there who has a similar set up as mine with similar overclocks? Is there a BIOS setting I may be missing that is bottlenecking my NB? Could I have hit a dip switch or something on the motherboard that is limiting it?

I'm really at a loss here, and my memory has been very poor ever since my doc perscribed me these thyroid meds, hehe.

Any help would be appreciated. If you have more questions, please ask, as I know this is kind of broad.