Both of the above.
Plus, it sounds like you're buying in fairly short. Maybe your style of play lends itself better to short stack play, or, maybe it's a time thing and your game changes as a result of getting slightly tired or something.
Or, maybe your style is better unread, so after you've been at a table for a while the other players read you better. Or, you start to get reads on other players but are doing the wrong things with those reads.
If the first, third or fourth are the case, change tables after you've gotten to, say, 50% up or more. If that doesn't adjust things for you, try just quitting after you get up.
But, also use the advice of the posts above as well.