1) You aren't getting your lua cache files from the server in time for the messages the server sends to you to work. (do you have server download enabled? If so, do you have your client allow inbound lua?)
OR
2) The usermessages are arriving out of order/too fast for your client to get them. Gmod usermessages are notorius for this.
Jay, if it ends up being #2, research ULib (or, ULX?) method for determining a player is ready for usermessages.
(I forget exactly..ULib has a function I think)
I'm guessing it's #1 though.