I think they (3cX) under estimated the amount of traffic a little. LOL.
I had the same problems as you, and mentioned this to admin and have not had them for a while. So I assumed that it all was fixed, or the problem had gone away.
In a few weeks time we are upgrading the server configuration and server. If problem then persists let me know. I hope it wont since not sure what else to do beyond that.