asterisk users - Sep 2011 - from asterisk 1.6 to 1.8 - sip trunk unreachable

Hi! 

I recently upgraded Asterisk from version 1.6.2 to 1.8.5

Now about every 10 minutes all SIP TRUNKS becomes UNRECHABLE for a few seconds
or minutes after become LAGGED and later become OK.

I have no idea of the cause of this problem.

With the version 1.6.2 all runs perfectly.

I can't say more because I have no idea where the problem is. 

Any suggestions? thanks
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