As I understand it because air is compressable you have to build up a certain pressure to overcome whatever frictions there are in the system, having done so the ram starts to move, at which point you have far too much pressure so it moves too fast.
I suppose with a really quick PID response time you could control the pressure fast enough, but you can't take the pressure back, you can only stop adding to it. So if it's too high there's nothing you can do without another mechanism like a brake or as I think you referred to, modulating pressure to the other side of the ram.
I suppose another way would be to have the ram working against a spring so the stiction is a small proportion of the total force needed. Of course the load may also provide this function depending on its weight. For example air bags in vehicles rise slowly, they have next to no stiction and a large mass to move.
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Rob