Problem with sip & iax not registering

i am having a problem with my sip & iax trunks to VOIP supplier intermittantlly not being able to register, they work fine for weeks on end but then i cant make calls and the log show registration attempts. i can ping the sip ot iax provider, reload SIP or IAX deosnt fix the problem same with restarting asterisk, i have to restart the server to get the trunks back. i have the same problem on a few servers on different sites.

any help on this would be greatly aperceated ad it is driving me mad.