Spiders do love to spin cobwebs in the tubes of those things.
My guess is that some compressed air would do nicely to blow things out.
That, and I think new ones run $30ish...
I'm assuming you've been to the troubleshooting section of their website, but if not, here's a link.
http://www.mrheater.com/troubleshooting.asp?id=24
Good luck, and let us know what you found...
Ted