PSG and their array of new talent after a close-season spending spree moved level with third-placed champions Lille, 3-1 winners at Saint-Etienne on Saturday, thanks to playmaker Pastore's 68th-minute strike.
The Argentine, a French record buy from Palermo, coolly found the net following good work from Jeremy Menez after visitors Brest had gone the closest at the Parc des Princes.
Olympique Lyon had reached the summit after five games with Saturday's 2-1 victory at promoted Dijon but surprise packages Montpellier now lead the standings again by a point after the 1-0 win on the south coast.
Senegal forward Souleymane Camara netted with a left-foot finish 15 minutes from time after a cross from Garry Bocaly.
"We are happy, we have done well even if it wasn't easy," Montpellier boss Rene Girard told reporters.
"This win is a reward for the lad Camara, who puts in top performances every time he comes on."
Struggling Nancy drew 0-0 at home to Auxerre in Sunday's other game to move a point behind beleaguered big boys Olympique Marseille, who have just three points from five games after Saturday's 1-0 home defeat by Stades Rennes.
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