I know it's naughty because this is thread necrophilia, but I've registered specially!
I have an MH-721 and had a similar problem. I've had my cutter for a couple of years now and it's always been brilliant (obviously, I've changed the odd blade but nothing else) until I had a jam the other day then it stopped cutting properly - faint lines and skewed lines.
So I took the advice in the thread - tightened all the screws (a couple loose) and then spotted a white wheel slightly off the track.
Hey presto, working cutter (now to sort those 30 odd t-shirts I have to do imminently).
So I'd like to say thanks, and often old threads are still the best!