If you look around, the usual number is 5%. The supply is too far on the high side. The only issue could be you are using the motherboard to measure this voltage, so you must use a Digital Volt Meter for a good reading.
But at 5.9 any decent voltmeter will show it's too far off.
I think you've found it.
In closing, such will work, but with the high overvoltage the parts subjected to such will likely run very hot and have a short lifespan.
I have a Dell dimension 4100 shich has destroyed 3 hard drives. All of a sudden you start to get an error message like it is trying to boot from drive a and there is no boot dist. So you go to set up and it seems to have forgotten the ide drives. So I set it up again and it might work and might not. After a few more rounds of this the disk will not reboot. Put the drive in another computer and it tells you the drive is bad. Thought it was the drive so I put in a new one. Same results after a couple of weeks. I checked the voltages from the power supply 5.9 and 11.9 which I think are in spec. Any ideas or similar experiences?