help! when i join a server theres a black grid type screen

[jonnysim214]

as i said above, when i join a server for maybew a minute and a half i get a black,lined type screen and when i try to close it (ctrl alt del) or anything else i cant my computers frooze.

other info

i run the server on my computer BUT this never happened with hack/mine and vanilla and running divinge rpg for 1.4.6