minecraft keeps crashing

Can direct connect to lan server but cannot see it on server list

I cannot connect to a LAN server from one computer to another!

World becomes semi transparent, non-liquid blocks will all assume the same texture, entire screen starts flashing. Will stop if paused using the 'esc' key, but restarts when reentering the game.

After a few minutes (2-5 mins) of game-play on both multi-player and single-player modes, the world will start flickering transparent, with dropped items still visible as well as block outlines.