Hi,
I experienced similar problems (with original adapter) and have come to the conclusion that the charging software of the N900 is broken from design. Also, depletion can suddenly go very fast without any obvious reason.

So I bought a cheapo wallcharger and a second battery. When the battery in the phone is too far gone, I just switch them and charge the empty one.

I like the N900 for its commandline. But it will certainly be the last Nokia I will buy.

Paai