Gearbox on 1999. 1.6 megane sport.

limited info generic answer can be a multitude of reasons --tranny + auto+ fluid low/dirty ---filter --torque converter-clutch pack--gear--tcm---standard +++fluid --clutch--linkage ++++engine--needs tune up --sensor[s]---++rear----failing--fluid----gear-----

I would guess one of the gearbox valves is falling put it into gearbox specialist before your stuck getting it towed will be cheaper over all good luck Kelvin

vacuum leak can cause surging.....

On the side of transmission

Range Sensor/Gear Selector/shift lever

after checking fluid level is correct, try this, to see if it improves...
into drive,let car accelerate without any gas(do not touch pedal)to max speed (about 30/35mph).stop,then with 1/3rd gas pedal,accelerate to max speed(do not move foot)stop,then foot hard to floor,upto about 60mph,then stop.then only using a max of 1/3rd gas pedal,drive for about 15miles.you have just reset the drivetrain computer setings....the car is fitted with a computer controlled fuzzy logic gearbox,it will try to make you drive as eco as poss,but if used harshly the gearchange can be harsh up and down.....
if driven gently 4th will engage at 43 mph,with a slight clunk.

Will the car move at all. If you put it in gear and let your foot off brake , does it move untill you touch gas , or does it jam up as soon as wheels start to turn. I am suspecting problem with diff.
Doubtfull about trans filter.

Get a second opion, but it looks like the thrust bearing is on its way out. That'll probably involve changing the clutch as well. Do it soon or you'll wreck the gearbox.

YOU might find its due to the Auto Matic gear box as when you slow down at the lights or brake hard to stop the auto box should brake free at about 600rpm as to allow the car to stop but the engine to keep idling,but the drive bands in the gearbox and the pump which operate them are not braking free so its like, in a Manual g/box driving and stopping but leaving the clutch pedal and not pushing it in so the car stalls,
SO you may find that the next time you are driving it and come to the lights just try putting it in NEUTRAL gear before you stop and try, if it ok then get the g/box serviced . RON

The gearbox does on have a dip stick it has a filler plug up top and a two part drain plug on the bottom of the transmission.

What you do is once the engine is warm, with the engine running you remove the centre plug and is the gearboxs level is low nothing much will come out. Some will come out as the tube gets full from normal running. If it is high the excess will come out and will stop at the ring.

I had the same problem with my gear box. What I did was remove the tube plug and drain all the oil out of the transmission. You then fill it with 3.5 litres of "Type D" transmission oil. Nothing else. Not Dextron anything or their equivalent.

In effect you have just carried out an transmission service.

I didn't even bother checking the oil level as 3.5 litres is the amount you need to put in and the gearbox has been running perfect ever since. The new oil worked a treat. Snappy changes and the clunking change with the light on stupidity went away.

Replace the speed sensor on the trans that will cause exactly what you describe is happening.