High CPU at every xx:55 (every hour)

Hello, i noticed that every hour and especially at xx:55 , my vps server cpu goes at limit for 2 minutes and then goes down...
Im using htop command but i didn't see any high cpu or memory process...
Any idea ?

Here are some screeenshots

 

I found the problem!
At the /var/log/cron there is a command "sync; echo 3 > /proc/sys/vm/drop_caches" which cause the problem.
I see this command every xx:55
Shall this command execute every hour?
How can i change the cronjob of this command ?

 

Can i change myself ?
How can i find this cronjob (at this file) ?

 

Hello, they suggest me to chhange the "echo 3 > ..." line to "echo 1 > .." in order to only free the page cache instead of also freeing dentries and inodes.

I found this:
To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
echo 3 > /proc/sys/vm/drop_caches

Before i removed the "echo 3 > ..." i checked my RAM. It had:
30717040 total, 25244176 free, 2631884 used, 2840980 buff/cache

After 5 hours i have (i had removed "echo 3 > ...") :
30717040 total, 21581380 free, 2164572 used, 6989240 buff/cache

Am I OK, What do you suggest?

 

After a couple of hours....

KiB Mem : 30717040 total, 244400 free, 2790844 used, 27681796 buff/cache

What do you think?
Shall i have to set again the "echo 3 > ..." or "echo 1 > ..." or im OK with these ?