If they're so easily manipulated, why don't prices just stay high all the time?
From what I know from my foray into day-trading, manipulating commodities is often not so much nefarious as it is the inevitable physics of markets. For example, say Goldman-Sachs has a bunch of major players as clients who see decline, they want out, so GS drops a massive short order....well, it's kind of a self-fulfilling prophecy. When that much volume sells, they actually cause the market to go down.
It's technically illegal to manipulate the market like that, but in reality it happens every day. The major brokers can't help but manipulate the market. It's just the nature of the beast. Those kinds of things, while manipulating the market, are temporary and won't have a lot to say about the long-term overall direction of the market. If there are other, nefarious, long-term, more massive scale manipulations, I don't know about them.
Now, there's also running stops, which they absolutely also do. This is pure asshole behavior and manipulation for the sake of crapping on the little guys, because they can. This also has little to no bearing on the overall market. It just wipes out 95% of the people who try to swim in that ocean with such massive sharks.