ok this is the way i have overclocked my n900.
install enhanced linux kernel for power users(settings) be careful while selecting this
and QCPUFreq from the app manager !

after installation open QCPUFreq and go the settings in the upper tab and select all the options (not sure about the last one , but i have checked it , dont know why :P )
and after saving it will give a pop regarding warranty .
after that it will take you back on the QCPUFreq main screen , and at the bottom you will see a slider , just move the slider forward to choose the suitable frequency .
i will suggest as many others suggested me to choose the ideal from the load presest ( in uppar tab same where you find the settings)
and there you go you n900 is overclocked

Try to install cssu it helped me when i overclocked my n900 using maemo-flasher,After i overclock it i was also having the same problem.

Just give it a try.

If you have a non-global firmware, you need to reinstall the correct mp-fremantle-pr package.

For example - for US firmware, the command would be:
apt-get install --reinstall mp-fremantle-002-pr

I would like to thank you all and show you my deapest appreciation for this post.

I recently had this issue when I was checking my n900 about product and it showed that unknow version. I had updated to the last cssu latest firmware and everything run smoothly only this matter

When i did Kernel refalsh and returened to it's previous condition it came back

thanks alot