Citroen C4 AL4 Transmission Issue?

Hi All,

My AL4 gearbox has been fine so far until last week, We had 2 very hot days and after a long hour of driving in traffic I finally got to my house leading road, it is a very sloppy and tough road, and when I decided to kickdown the car to give some more boost to go up, I experienced 3 times a slippery transmission shift where you hear the transmission slipping before going down from shift 2 to shift 1, the revs go up and followed by a bang. IT never happened to me before. Nothing was displayed so far on the screen and I didn't experience it since 5 days now. I am sure it has to do with the oil and temperature but I would really appreciate if you can advise what is happening? should I change the oil or I have a defected gearbox now? As far as I know Citroen garage changed the oil as stated since 18000 km...I also showed it to Citroen, they stated that I need an overhauling for the gearbox which I didn't believe much honestly.
I will be waiting for you advice.

Thanks,

Mark