If you have a multimeter, check it at a molex NOW.
IF the reading is correct get a new psu
IF the reading is NOT correct, you may have a mobo problem
Are you using the mobo headers to power anything?
Do you have any high amp fans plugged into mobo headers?
How many devices are you running in that pc?
My old VP6 mobo had a similar problem for a while, although I did NOT find the cause- I moved form one case to another and a week later the 12v was rising with all other voltages ok.
I could not find a problem, but after two weeks my 12v was around 14v and I started to smell something funny. Frequent crashes were starting to occur.
A trace on my mobo burned out
and the board was never the same; RIP VP6.
I would try to find exactly what the problem is fast, my friend: that is too much voltage to be going to the motherboard.