The problem is fairly clear -you both need to replace your POSburg with a nice Weber 32/34DTML.
Your Pierturds are over-complex and to the point that they're worn out, which is why they're dicking about.
So, why a Weber?
They're very simple.
They're more efficient than the Pierburg, you're probably going to get 5-10MPG extra.
They deliver a better mix of fuel to the engine - swap out the carb and bingo - more rev-happy, powerful motor.
Just make sure you use the original inlet hoses and the warm-air flap, or you *might* get carb icing from time to time.
Oh, the whole wax-stat thing - you can get replacements from GSF, but I wouldn't bother..... provided you can use a manual choke..... which you can, right?