Forget about a new sim card, that advice will not solve your issue, the N900 will just tell you the sim card is dead and continue booting. It's completely unrelated.

Flash your phone and do not make any modifications to it. None. Do not be tempted, just flash back to factory default. rootfs & eMMC. Don't try to restore any backups, nothing. Factory. Default.

Once you've done that, use the phone for a while, see if it still has the same problem.

On the previous page you said you had symlinked some directories but weren't sure if you had done that before or after the problem occurred. You need to be certain, so don't symlink anything.

Once you've done that you can then firmly establish any possible hardware fault vs software.