The way you have to look at it, having a stab in the dark hoping you did the solution could cost you more money than good quality diagnosis.
It's worth a shot putting the original style manifold back on.
Speaking from experience, you can't run an old Audi on a show string. My A6 has only covered 341k miles on the original engine and gearbox because of thorough maintenance. Including preventative maintenance.
Old Audi's get cheaper to buy but bills always remain high. That goes for any German car.