How to find source of system running slow clock?

I've a problem on one of my Gentoo server. After some time, the clock starts running very very slow. It takes about one minute for one second to go. The major problem is that not only the clock is impacted but the system is running slow as well. If I run top, it takes more than 10 seconds to open. A sleep 1 takes about a minute.

There is no load on the system, almost nothing is running. I'm running the kernel 4.0.5. The hardware clock is running fine.

What could be the source of such problem ?

Here is the clocksource information:

# cat /sys/devices/system/clocksource/clocksource0/available_clocksource
refined-jiffies jiffies tsc
# cat /sys/devices/system/clocksource/clocksource0/current_clocksource
refined-jiffies

Changing the clocksource to tsc fixed the problem.