It's possible the fan has partially failed, so that it is needing more power than your board is supplying. Is it making extra noise?
You might also try disabling any fan control settings in the BIOS, so that it runs at max power.
You can even just run the fan off of the PSU if you can convince the board to allow that. You'll need an adapter of course.
Are you using the fan speed controller that came with the cooler?