My first reaction to any trouble with the N900 is to hit up "Power Search" at these forums.

9 out of 10 times someone's ran into my problem before me and I get at least some pointers as to what might be going on.

Also, I don't know if its me living in Finland, but I have a Nokia Care Point somewhat near by (about 40mins away), and they're always more than happy to have a look at my device. I've always been very satisfied with the quality of their service (taking it in and the way they've repaired my phones).

I think in the world we live in it's the consumer that needs to take a very active attitude in just about everything and anything. If I had been in your shoes, I would've searched the forums and spoken to someone at my Care Point about the problem. I couldn't go 4months with something on my mind about my N900.

Hope it all ends well for you never the less!

EDIT: Oh, and because your problem sounds like a software problem, then I think there's even less people that know how to help you out at Nokia. Break a physical component and they know what to do. Helps to think about the N900 as only a "semi-official product" of Nokia.