This is quite urgent as I need a working phone tomorrow.

Today, during an SSH transfer, my N900 rebooted. Since then (~ 2 hours) the phone randomly reboots. It happened originally when using the DSP profile, but after using the phone ~10 mins, it reboots once again (so without any custom kernel settings). On boot "an unexpected reboot" is shown.

/proc/bootreason shows "sw_rst"

What might be the reason?

have you checked you battery contacts? I has once similar problems. bending the contacts a little bit solved my problems instantly.

How exactly should I check & bend them?

I odn tknow how toc heck them but bending them is easy. Take any pen/pencil and press the contacts of each battery terminal, so that they get a little closer to each other.

re battery : scrape with a pin i.e the contacts and if really needed pull em a iittle bit
as of me my other n900 has that reebot problem mainly its batry contact issue and sometimes a reflash i.e now a days i use backup menu

Battery contacts might be.
Check if battery feels abit loose, then bend the lower spring contacts carefully.
Check if contacts are clean (neither gray nor greenish). Clean with (isopropyl-)alcohol.

But I am afraid it might be some file corruption (you know not to check with fsck on-device?).

what are the "lower spring" contacts?

So far I cleaned the contacts and it seems to have fixed the issue. Thanks!

Those flat batt contacts at the outer edge. (they are not really 'springs'!)
Just get a clock screw driver (or I also use a nail shearshear sometimes ) and bend them inwards.

If cleaning fixed temporarily you should do that at at next occasion. Will avoid future problems..

Well, an aftermath of my problems with rebooting.

In braces the reproducibility
1. Sometimes (2/10) the hildon desktop mixes the orientation, staying in portrait resolution. The layout is rotated, though. killall hildon-desktop helps
2. Once: image-viewer takes > 20% CPU, so does hildon-desktop. device sluggish. killall hildon-desktop helps
3. (100%) a particular image causes the device to crash (reboot) when zooming in. Once even switching the photo caused a reboot
4. During mass storage mode, the device exits the mode and freezes, not reacting to the power key. (once)

Does it sound like fs corruption? Or may it be something else. Will fsck suffice or should I rather do a complete reflash?

/edit: tbh, I never did a fsck on my phone. Is there any way of doing a rootfs fsck on phone than using some rescueOS?