Temperature drops to zero at idle

Got a friend with an 06 Altima 65 k on it. Says temp gauge for no reason drops to zero only at idle after few mins sitting. Everything is perfect has heat never overheats etc. put new temp sens switch on still same thing. Only does sometimes after idleing a few mins. Any ideas what to try next I have no ideas. Also when this happens tach revs up like to compensate. Thanks.