Hmm
it could be a few things...
the card may not be seated properly...you say it connects and then doesn't ...and then does...that may be as simple as a piece of thin folded paper placed over the sim as many have done...in a variety of ways..to add enough pressure to enable the sim to contact the pins properly...
Hmm..could be something as rudimentary as that...a physical fix.
or it may be internal....
or it could be the service area...
anyway...Best suggestion is to definitely read ...esp. read everything you can on the topic and process of elimination utilizing the easiest and least invasive methods 1st and narrow down the list of probable causes and apply the fix that works. You know best how and what your device is performing ...better than I or any one else....

I'll post in this particular post a few...(patience..it may take a bit)...threads where you may find the particular answers to your dilemma ...and/ or you can post probs..questions and screams for help
I really suggest you use sub-forums to post in...if you are asking what everyone knows is a common question...they'll direct you to the section where that issue lies....considering you are asking about the n900....the subforum and freemantle forum are the places to shout out.
Asking a specific question in general forums isn't a grand idea...since your question will very quickly get buried and lost ...in the place no one would look for such a question..And remember...if in doubt...and you can't get an answer out of the search engine......google is your friend....
Anyway here is a thread concerning the sim card and common probs not being recognized.

http://talk.maemo.org/showthread.php?t=36447