Chain skipping on MTB I've had my Trek 3900 for about a year now, and I replaced the chain about 3 months ago, but otherwise it's running with the same parts it came with (not counting tubes and tires). 
I had to replace my chain because it had gotten really rusty from riding in rain/mud/water on trails, and had started skipping.  

My problem is, whenever I ride, if I get above or below a very light pressure when pedaling, the chain skips. I was wondering if this could be caused by my chain, which has gotten a little rusty but not as stretched as the other one was, or if I might need a new cassette?  

I just readjusted my derailer, but it still skips. 

Does anybody know what could be causing this?
When your chain "stretches" beyond a certain point, typically .75mm per 4 inches links, it will begin to reshape the teeth on your cassette and chain rings. 

At that point, replacing your chain will not help unless you also replace your cassette, and possibly your chain rings.

Check the answers on this question to see how to know when your chain is worn.

If your chain measures beyond 1.0mm per 4 inches, then you must replace chain, cassette and chain rings, or it will skip as you are describing.