With 16 points from 11 games, they lie seventh in the standings, five points behind Brest, who claimed top spot on Saturday with a 2-0 home win against St Etienne.
Stade Rennes dropped to second after their trip to champions Marseille, originally scheduled for Saturday, was postponed twice because of heavy rain in southeastern France.
That weather also forced the postponement of the Monaco v Girondins Bordeaux match.
In Montpellier, PSG went ahead in the 37th minute when former France international Giuly fired home from close range from a Nene corner.
Montpellier, now sixth on 18 points, piled on the pressure and were rewarded three minutes from the interval as Chilean midfielder Estrada collected an Olivier Giroud cross to beat Apoula Edel with a low shot.
The visitors came close after the break but Guillaume Hoarau's header smashed the post in the 67th minute.
Valenciennes and Lille drew 1-1 in Sunday's other game.