Game is unplayable because of lag. Someone explain.

Someone tell me why I get these random, several minute bouts of lag, primarily when I'm in a losing game. Game runs fine for the first 15 minutes, then, I get horrible jitter and my ping spikes constantly, making the game freeze me in place constantly. Someone explain to me why this is happening now, of all times, over all the years I've been playing this game. This is fucking stupid and is making me lose games.