[The Solid lad]

"I've narrowed the cause of problem down to the server.cfg file, since when I start the server without it, this problem isn't present."

^Meaning that it is a server side issue, and the cause is a command it that server.cfg file, since when I don't let the server execute it, the problem isn't present.
Also, this doesn't happen on all servers, just a select few, my new one included...which makes it even more clear that the problem is caused by the server.
And no, I'm not confusing framerate with tickrate.