I have the same problem. I have tried three different N900 and experienced the same problem with all of them. I have only brough contacts and calendar from old N900 by using vcard 3.0 and mail for exhange sync (tried to avoid bringing the problem to new device).

The installed softwares in my latest device are:
-cpumem-applet
-Desktop Command Execution Widget
-camkeyd
-DrNokSnes
-Headphone daemon
-Hermes
-Ines
-OpenSSH
-Rootsh
-TouchSearch

Only widget I have installed and I am using are TouchSearch, Desktop Execution com. widget and Foreca Weather.

I have also noticed that when the phone freezes after call the mail client and alarm clock doesn´t work. I have already been late from work five times becouse of this.