Could be many causes, but same sollution. First try a good oil additave to swell the seals (I recomend Lucas Oil). If that doesn't do it, it is time for an overhaul on the seals. Also, visually check that no hoses are loose, they do wiggle a bit when you drive.
The automatic transmission fluid that a Peugeot 306 holds is 2.4 liters.
Having too much fluid will cause your transmission fluid to look foamy
Yes - could cause fluid to foam and not lubricate properly
the transmission could be going out or slipping or your transmission fluid could be low
It could. When overfull the fluid can aerate, which will cause slipping and a loss of lubrication.
check transmission fluid for correct amount or being dirty if fluid is ok your transmission is wearing out
Yes, that can cause the fluid to foam up and leak out of the vent.
Yes. Overfilling the transmission fluid may cause problems within the transmission. It could cause abnormal operation within the transmission because the fluid is not performing correctly. Transmission fluid does not compress, while air does and as a result of overfilling, the fluid could become aerated. If you're having tranny trouble, it's best to leave the fluid alone and visit a tranny specialist. They can remove the excess fluid AND determine if any damage has taken place.
yes it would cause the gears to slip more or less
shifter cable, low transmission fluid or faulty transmission.
No the transmission will not overheat, the wrong type of fluid could just ruin it the transmission. Same with also using too low or high amounts of fluid. That's why they recommend using the right type of transmission fluid.
IF you have over filled the transmission this will cause it to bubble, this could cause serious damage, drain and refill with the proper ammount.