Nat issues

This is my story: I had a SM 5.7 registered in AP1 doing NAT very good, but I had to change it to AP2 because AP1 were suddenly full of intereference, so I change in the SM the NAT public address because AP2 is in a different network, since then the NAT is not working good, every 5-10 minuts seems to hang up and the users behind the NAT lost conn with the network (I still ping the SM to its rf ip address there is no interference or lost of link). I reset the SM and the NAT work ok for another 5-10 minuts, so I have to reset it again and again making the users very pissed off.
I defaulted the SM and reconfigured again, not worked, I downgraded the unit from 6.1 to 4.2.3, not worked. I upgraded to 6.1 again, not working. Anyone have and idea here?

Thanks in advance