We have a client with a 3825 router with an NM-4T. They are using port 0,1,2 in a ppp multilink bundle. They having slow bandwidth issues and the all of the ports on the NM-4T are showing carrier transitions, even port 3 which does not have anything connected to it. They have checked with the line provider and the provider claims the lines are clean. The slowness issues seemed to go away after rebooting the router, but the problem returned within two days.
I have this problem too.