Is there possible to go back to WM5 if I upgraded my 838pro to WM6 already?

hi everyone,

I just want to ask, will there be any problems if I flash the ROM back to WM5 for my Dopod 838Pro (WM6)? Cause I've found some serious problems with the phone function.

Here is the case, see any of you have the same problems:

When the phone is on StandBy mode (Screen off, phone is still working), incoming call is working fine but a little bit delay (says - 2-5 sec)

But when the phone is active (Screen on, nothing to do with it), no incoming calls can be called in (they just keep ringing with their handset, but my phone is not ringing at all), because I have voicemail, the call will be forwarded to the voicemail because "no one" answer the phone.

Does any of you guys experience the same problems from 838pro/CHT9000 with WM6?

Thanks for your kind attention.
Raymond