I have the same problem. Bought a MAC 7-42 and after a few cuts it started to pull to the left before the bar got stuck. I noticed that the chain had become loose, which if you digest every word in the manual it does warn. I tightened the chain, but it still cut to the left, so I took it back and got a replacement. Same thing happened with the replacement chainsaw. I think what is happening is that if you don't catch the moment the chain starts to become loose as it expands, and continue to force the chainsaw to cut, it knackers the chain.
You can try and file the chain so that it is even on both sides. Or buy a new chain. You might even need to get a new guide bar if it's become damaged by making curved cuts.
d4, September 2009