MariaDB random stuck on insert

We have database with low amount of inserts/updates and much more reads. Sometimes it operates normally for months, but sometimes it gets stuck on insert into main, quite big myisam table, for hundreds of seconds. Other connections are waiting for this one to finish. This makes the whole database stuck for quite a long time. A few days ago, I couldn't even kill the query. In processlist, its Command was "Killed". Other inserts and updates' State was "Waiting for table level lock". The processlist was over 1000 queries long, in comparison to usual 10-30. The only thing that always helps is restarting the database server. We have Icinga for monitoring, but I don't really know where to look... I suspect the disks or wrong configuration of virtualization can be the reason, but I have no evidence for this claim. It's just my instinct. Any ideas?