I had a Silberpfeil (S2 19 Deluxe) for a couple of years before I got my Speed 3 21. Don't remember details (does it have ring lines on the tips? Or was that just the Psycho3?) but I'm sure the basics apply...
Your profile is wrong -- you've either tuned out the reflex so there's no shape to the canopy itself to stop it over-flying, or the bridle has shrunk to the same result: bad profile.
1. Make sure your bar is actually tuned, including the right mount of throw on the depower rope. I'd do the tune right up to the bottom of the mixer, so include the pigtails.
The mixer adjusts the profile of the wing. The assumption with a zero test is that the bridle lines haven't shrunken terribly, or that your SPL (the mixer pulley line) is still within spec.
2. If you haven't replaced the SPL, you should. IMO make your own, use light dyneema <2mm (can't remember the sizes) and make it 10cm longer ie. Speed3 SPL are 100cm and 120cm, so make them 110cm and 130cm. Check the line plan for the Speed2 though. Nice eye splices and a Brummell, and a Zacher knot for the knot end.
[There's also a trick that works by adding a second knot the C-line -- there are threads at kiteforum about it, I think foilholio is the author. I think that's about returning throw to Z but never bothered with it... check it out and add the second knot now -- you'll need to add about 5cm to the uncut length.]
3. Mixer text to zero, fly it. Don't forget to line up A and Z, then do C first, not B, because adjusting C adjusts B by half.
If it still isn't right, try:
4. Long mixer test. (this is IIRC)
Get the line plan, work out the differences between A1 and B1, B1 and C1, C1 and Z1. It's something like 10cm, 20cm, 40cm.
Secure the TOP of the mixer, stretch out the bridle lines and compare A1 and B1. Measure and make a note.
B1 and C1 -- measure, note.
C1 and Z1 -- measure, note.
The differences should be the same as the differences in the line plan, within like 10%.
If they're not, add a pigtail to C above the pulley -- IIRC. I forget this bit exactly, there's threads on KF again with the details.
If that doesn't help, then you're going to have to meddle with the mixer.
In all reality, the S2 was a simple-minded beast that always had a tendency to overfly. And a foil isn't a tube, if you let it shoot to the zenith, it's going to overfly. Maybe you're just over-reacting to it's natural character ... and being too heavy handed on the bar.