Server lagging and occasionally crashing.

[Mycaelis]

I've got a server with a 3rd party hosting company and regularly the server is lagging and occasionally crashing.
This can occur with any number of players on, but my server only has a max player limit of 18.

The only information I get from the console logs before the lag / crash of the server is as below:
[QUOTE]08/10/2014 - 13:20:25: ServerLog: Round ended.
08/10/2014 - 13:20:25: ServerLog: Result: innocent win.
08/10/2014 - 13:20:34: Dropped XXX from server (XXX timed out)
08/10/2014 - 13:20:37: STEAMAUTH: Client XXX received failure code 6
08/10/2014 - 13:20:37: Dropped XXX from server (Client left game (Steam auth ticket has been canceled))
08/10/2014 - 13:20:49: STEAMAUTH: Client XXX received failure code 6
08/10/2014 - 13:20:49: Dropped XXX from server (Client left game (Steam auth ticket has been canceled))
08/10/2014 - 13:20:54: STEAMAUTH: Client XXX received failure code 6
08/10/2014 - 13:20:54: Dropped XXX from server (Client left game (Steam auth ticket has been canceled))
08/10/2014 - 13:22:43: Dropped XXX from server (XXX timed out)[/QUOTE]

I've tried various server.cfg setups, I've also tried tickrate 33 and 66, but nothing seems to have any effect in stopping the problem.
Has anyone else encountered this problem and found a solution to it?