I have installed the Enhanced Power Kernel and I sometimes overclock the device to 800Mhz using cpufrequi or qcpufreq.

My problem is that after i cannot return back to the 600Mhz setting and I have to reboot the device. It does not accept the 600Mhz mark although it accepts the 550Mhz.

any ideas how to fix this?

install the Enhanced Power Kernel (settings) and kernel-flasher-maemo.
lunch xterm and type : sudo gainroot
than kernel-config limits 250 600
you can replace the last 3 numbers with the frequencie you want, here is a list of the supported frequencies (for both numbers) : 125 mhz, 250 mhz, 500 mhz, 550 mhz, 600 mhz, 700 mhz, 750 mhz, 805 mhz, 850 mhz, 900 mhz, 950 mhz, 1000 mhz, 1100 mhz, 1150 mhz .
i'll suggest that you lower the voltage after the sudo gainroot command, by this command : kernel-config load lv .