FWIW, I had similar problem with a new sealed in box N900 that I bought off of ebay not too long ago...at that time I did not have an external battery charger.

Seller asked me to return the N900 for testing. After seller tested it, he sent me a message that there was hardware problem and would send me another N900. I received another new, sealed in box, N900....

In meantime, I bought an external battery charger and used it with another N900 that I bought and had similar problem. After charging battery in external charger, the N900 booted up normally and works well.

So, it's clear to me that N900 is sensitive to battery charge level, if it gets too low, you will have problem booting and possibly recharging battery inside of N900. If battery too low, then much better to charge it in external charger.

Hopefully, GalaxyBat, your new wall charger and batteries will solve your problem. Good luck!