I think I know what's going on. I had a similar issue when I rebuilt my front end. I changed everything except the torsion bars - thought I'd save some money. Mine was doing the same thing, settling back down onto the bump stops. I think the old torsion bar metal, after twisting thousands and thousands of times loses it's "spring" in the initial twist. You need to crank them up more and more as they get older, to get to the "newer" metal that hasn't been previously under strain and can therefore give some "spring" back the other way.
When your adjusting them on the jack you probably only need a few turns to get to the desired height but your not hitting the "new" metal. There's enough spring to hold the car up, but once you start driving they settle down again.
When you adjust them with the weight of the car on the springs you probably turned them more to get to the right height and therefore put the bar under more torsion than it's "usual" position and therefore it holds it's height.
You may be thinking now, what is this bloke talking about? You may be right - I could be talking bollocks. I just think of the old bar like a piece of elastic that's been stretched too many times - you need to shorten the elastic to get some tension. With the bar it just needs more twist compared to a newer bar.
I cranked the adjuster on mine a few times more (car still on the jack) and it's been holding fine ever since.
