Montpellier conceded three goals in eight minutes as it lost 4-2 at Evian on Wednesday and conceded ground to defending champion Lille in the French title race.
Montpellier had led 2-1 and its surprise defeat means big-spending Paris Saint-Germain can move three points clear at the top by winning at Saint-Etienne later Wednesday.
Although Lille moved within one point of Montpellier after drawing 4-4 at home to 10-man Nice, it surrendered the lead three times in an absorbing game which saw Nice equalize in the fourth minute of injury time.
Lyon, meanwhile, was full of confidence after winning its last four league games but lost 1-0 at Valenciennes and missed the chance to overtake both Lille and Montpellier.
Montpellier took the lead early in the second half when Younes Belhanda scored from close range after goalkeeper Stephan Andersen had saved John Utaka's shot.
The lead lasted only five minutes as veteran midfielder Cedric Barbosa equalized against his former club, reacting quickly to clip the ball over keeper Geoffrey Jourdren.
Two minutes later, Giroud restored Montpellier's lead when he ran onto Belhanda's perfectly weighted pass and lobbed Andersen expertly for his 13th goal of the season.
But then Montpellier's defense fell apart as striker Saber Khlifa and defenders Cedric Cambon and Brice Dja Djedje all scored. The loss means Montpellier heads into the winter break without a win in its last three league games.
In Wednesday's other results, it was: Auxerre 2, Dijon 2; Lorient 0, Caen 0; Rennes 1, Bordeaux 0; Sochaux 0, Ajaccio 2, and Toulouse 0, Brest 0.
At Lille, Aurelien Chedjou's deflected strike put the champions in front, but Renato Civelli equalized in the 16th minute when he headed in David Hellebuyck's corner.
Joe Cole almost put Lille back in front moments later but his spectacular overhead kick went just over, and Nice goalkeeper David Ospina kept out Dimitri Payet's shot as Lille poured forward.
Cole restored Lille's advantage in the 27th with a superb strike, the former Chelsea midfielder swapping passes with defender David Rozehnal and hitting a first-time shot flew into the net.
But Nice stunned the home crowd with two goals before halftime, with Eric Mouloungui setting up both.
Mouloungui broke free down the right and dribbled his way into the penalty area, waited for defender Franck Beria to commit himself and played the ball calmly to Dja Djedje, who scored from close range.
Mouloungui then turned Chedjou inside out, and hit a fierce shot that 'keeper Mickael Landreau could only push straight to Dja Djedje, who tapped into the open goal.
Lille almost equalized straight after the break when Ospina beat away Eden Hazard's strike from outside the box, but then Mouloungui surged forward and hit a curling shot just over the top left corner.
The Gabon international was proving impossible to stop, whatever side of the field he found himself on.
In the 63rd, Mouloungui gathered the ball on the left, cut inside, poked the ball through Rozehnal's legs and smacked a rising shot just over.
But Mouloungui then got a second yellow card in the 69th for fouling Florent Balmont and was sent off.
That spurred Lille on, and Hazard equalized in the 76th when he collected substitute Ludovic Obraniak's pass, turned his marker and fired into the bottom corner.
Balmont though he had scored the winner in the 88th, with Obraniak again the provider.
The Poland midfielder clipped a delicate pass over the defense which Balmont controlled on his chest before finishing with a crisp half-volley.
But former France defender Francois Clerc made it 4-4 with virtually the last kick of the match after Lille failed to clear a corner.
Rennes bounced back from a spell of bad form with Jires Kembo-Ekoko getting a second-half winner.
Benjamin Corgnet and Brice Jovial put Dijon 2-0 up, but Anthony Le Tallec and Issam Jemaa scored twice in the last 10 minutes for Auxerre.
Ajaccio remains in last place, three points behind 19th-place Sochaux, but a second straight win gives the Corsican club some hope of avoiding relegation when the league resumes on Jan. 14.
Marseille won 3-1 at Nancy on Tuesday.
