WMI speed degrades with time

Question

We have server with several services, working there. They all write different perfomance counters and during last time (half a year maybe) I see that WMI work degrades with time:

• we restast server, everything's fine;
• services work, write there counters for a pair of month;
• WMI starts working slow - window "Add counters" in PerfMon takes about 7 minutes to collect list of counters data (categories, etc.), WMI instances (started by monitoring solution) takes about 10-15% CPU utilization;
• we restast server, everything's fine.

OS: Windows Server 2012

restarting server - helps for a limited time; checked counters data integrity - ok, no problems.

Where should I look first? How to fix this degradation?