I'm not in Japan but I have some suggestions that maybe able to help you. You should check if Bmobile doesn't work on pure 850MHz band. N900 is a quadband device but it does not support 850MHz or any particular country specific radio bands such as China which uses some fancy 400+MHz band.

Japan as far as I know used to not provide SIM services. They used to have similar designs as US did with CDMA. However I have heard that they are phasing that out in a bid to slowly conform with the majority in using SIM (thus GSM) services. The only thing however is the radio bands that each operator is allowed to operate on and thus comply with the national regulatory authority.

The GPS signal issue I think is now a widespread known problem if you're using AGPS. Though there has been no concise proof of any probable findings but the bottom line is that nokia's SUPL server is playing up. Many have been advised to use google's SUPL server and has had yielded great success.

To change the GPS server (if you're using AGPS) go into: Settings -> Location (under Connectivity) -> Location server (under Network positioning on the very bottom) -> change to supl.google.com (instead of supl.nokia.com).

If you're not using AGPS mode but only the internal GPS, it will take a fair amount of time to get a proper lock on your current location. Variations under weather and potentially under various areas (such as under a carpark with a roof, shopping centres, etc) can influence the accuracy and thus delay the amount of time for the initial tracking.