with Stock kernel, PR1.2 and no other special software running:

I notice there is some sort of battery capacity melt-down when
the n900 ambient temperature reaches a critical point (?)
-and- is running wifi and in particular web browsing (microB).
In that instance the battery just wilts away in a matter of an hour or so.

Turning the n900 off and letting it cool before turning it back on
seems to help fend off this moment.

I have just upped to PR 1.3 and haven't seen this yet,
but that may only be due to habitually being wary of getting the unit overheated.

If you are connected through a USB port or it is otherwise charging
(you said you are using PC suite - I assume windows and cable)
then I would suggest you make sure it has some sort of fan or other means of keeping the device from cooking off