Hmm.. boredom or writers block? I do know what you mean, and I have two ways of solving it:
a) start another piece that is never intended for the light of day, it should be the furthest from the style that you're writing and it should be fun, fun, fun. It could be an autobiography (well, you'll need it when you're rich and famous), it could be a childrens book with disturbing characters, it could be a trashy sex novel, a gripping war story - anything you like, anything at all. In this story you will just write what you want, never correct your own grammar, and laugh to yourself when you read it back.
Why do this? Well, because I think that 'boredom' isn't boredom, it's a kind of brain-fatigue in the creative centres, where you're trying incredibly hard to produce the best that you can. You're calling up and streaming an incredible amount of consciousness, trying to filter it down to words... Of course it's hard, and eventually (after 500 words, in your case) your logical brain gets fed up trying to do this for you, and you get 'bored'. It's almost as if the logical brain is saying to the creative "Come on! I need ideas, dialogue, action!" and the creative brain is being stifled by the continual demands.
So, to free this up, you write without pressure just 'aving a laff. The logical brain sits to one side, and you write for the pleasure of it, nothing more. In my case, it actually gave me ideas for my major opus as I did it, and I found myself wanting to go back to it to get them down. In this way, I found I wrote more and more words. I haven't actually visited my trashy novel in ages, I think I've just trained my brain...
b) Stop writing and play 'Hearts'.