Hi There, 
I assume, that this has been discussed here already pretty often. However, it seems to be, that the "stable flag" is only assigned to relays with a static, or at least long lasting, IP address. It also seems to be, that the stable flag is mandatory to get the guard flag. If so, that would mean: Dynamic address = no chance to obtain the guard flag. Is that the truth? 
Tnx 
Mike