some time my n900 hangs when say applying new theme or restarting hildon servies as tuning. I am attaching a screenshot of the same for reference.

only reboot solves the issue. Is there any work around for this?

Have you overclocked or done any other 'low level' tweeks? What apps have you installed? Especially those related to kernel/speed up/tune up etc?

I have overclocked mildly, 500 to 805, smart reflex on, cggroup patch from one of the threads and swappolube.

have tons of apps, this happens probably with heavy i/o with theme changer changing theme.

I would disable smart reflex and try without it, it can cause severe instabilities.

I'd put money on one of these hacks/tweaks etc is the cause of your problem.

As to which one? sorry - my way of fixing would be to reflash and reapply the tweaks slowly, and monitor the 'performance' of each one. But that's a heavy handed approach.

But a wild guess - try turning smart reflex off and see if that fixes it. My N900 hates smart reflex....

let me switch off the smart reflex and try it out. might be issue with that. though i can say it does improve my phone's battery life.