I'm not sure. Does it happen regardless of rpm? In other words, if you are cruising along at the top of 3rd gear do you get the stumble at 20psi the same as if you are accelerating from the bottom of third? What is the history of your valve springs? Maybe something like high exhaust pressure is pushing open the exhaust valves and giving a nice dose of EGR?
One option is to consider a well-fueled 18 psi decent abuse on a vanagon trans and solve the 20 psi stumble by limiting boost to be below that.
Point well taken. The tranny I have has served me well and I hate to thrash machinery. However, I need to see what this thing can do and I'd rather do it with this one than the new rebuild. I very seldom put the pedal to the floor and don't intend to do so with the new transmission. However, ever since the rebuild of the motor, I've had this feeling that I'm missing something. It just feels like its struggling more than it should - hard to describe.
20 Psi is a rare beast and only shows up for a split second when I'm really into it so, no, it only happens when I hit between 18-20 psi.
I have a "tiny tach"which works well but not too good for a quick glance I'm guessing RPMs are probably in the 35-3800 range at that point. It then stumbles immediately dropping to 11 PSI - I back off, and all is well again. This morning on the way to work it did do something it has never done - it felt like the turbo had just quit or I was running out of fuel. I was doing about 55 or 60 and when I gave it even a small amount of fuel, it just went flat - did this several times at ever bump of the pedal. I immediately headed for the nearest exit and when I got off, it was fine. It took a few blocks to find my way back to the freeway and it never happened again. It is still doing the 20 psi stumble and it must have something to do with the major increase in tension of the star-wheel spring as that's about all I've changed (last weekend).
Maybe now that my boost leaks are fixed, I should back the spring off?
7 psi is cruising at 60-65 so it's not that big of a deal. I'm just trying to find the limits and they seem to be much closer than I think they should be. That said, Theman's 1.6 has some serious porting and other mods and his car probably weighs half as much as my van. It's certainly more aerodynamic

. Pitting his 30 psi and 800F Egt's against my brick is not a good measure.
Oh, as to the valve springs, the head is was a brand new Kolbenschmidt, I believe there's not much chance of an issue but I spoze anything is possible.
As I'm writing this, I recall that the Tiny Tach may record the highest RPM, I'll do some checking and see if I can get it to reveal itself. Possibly I'm floating the valves without realizing it.