As far the 'get stuck' part is concerned, a N900 in mint coditions - i.e. just flashed, no extra app/widget installed - should not have too many hog problems, except perhaps flash running on some website. I never experienced missed calls with the phone 'clear'. Obviously the 900 is born to install stuff, but it is not resilient to much of them!
I made a (technical and boring, very long - http://talk.maemo.org/showthread.php?t=71115) post on the matter, since I like 'geeking' with my n900 but at the same time it is my production phone and therefore to loose calls IS NOT AN OPTION.
Usually it happens when some process hogs the phone and/or you have a very high swap activity when a phone call come in.
If you are not afraid to change some critical parameters, you could try to implement some of the modifications I proposed.
Swappolube could help, but IMHO only in the short term and from a 'feeling' point of wiev so definitively worth trying, other scripts, well... I think that it is better to reboot the phone every night. Alarms will turn it on anyway when you wake up in the morning!
What are your standard use-cases? Did you notice some patterns that lead to missed calls?
I had phone rotation disabled in order to avoid the extra stress related, and it helps a lot to not to miss calls.
Now I have it re-enabled again after all the job reported in that post.