Sounds like a high resistance which you could temporaly improve and solve...but it will happen again here the explanation:

I would say you made pressure to the back of the pcb board by cleaning the sim slot und putting back the sim.
After that all seeming ok but then you made pressure to the front of the pcb by typing on the keyboard, all went bad again due to a half broken connection and high resistance.

This could be an explanation and it is known that the N900 keyboard design is not good for the pcb and heavy typing will lead some day to this problem.
So probably you have now bad balls under the modem or some broken wires or vias in the board. If it is only the modem then it could be resoldered, when it is the pcb board you need a new one.

I think you will also have a higher battery consumtion and you would be able, if activated, to see many kernel messages from syslog regarding the modem.
And the behavior will get worse by time of usage.

My tip is try to avoid using the keyboard when it is not really necessary needed so when your at home do the console work over ssh.
Or for you as an artist and good in handcraft you could design a new keyboard, stabilize it or protect the pcb from pressures.

Source another N900 as long as you can

Edit: Also swollen batteries are PCB killer and old batteries could also play a small role for the modem.