Call of duty Black ops zombies glitches?

Sometimes (well most of the time) on Zombies i'll be on and the sound will be out of time for example if we get max ammo or insta kill then it will not say "insta kill" and later on in the game it will play them all at once and sometimes when i buy a second gun it doesn't show up but does to other people? and after a game it's on a blank screen for about 1 minute instead of going onto that movie thingy it's very hard to play like this any idea why this is happening?