>> does the stability of an ADSL max line depend on the throughput speed or the speed at which the line in synchronised?
In short sync speed.
But theres a lot more to do with it than that such as the SNR Margin at that particular point in time - It can fluctuate.
The higher your SNR, the better it is, therefore the more chance of you syncing at the higher speeds.
When your SNRM drops too low, thats when errors start occurring and the line can become unstable and drop.