We'll twin turbo systems are a nightmare to pin point a problem with... All the associated pipe work can be a bit of work.
You really need to get a gauge on and see what boost it's making, you may see low boost, boost needle flapping around, no boost, or perfect boost... Each one of these wi point you on a certain direction..
It may not be boost related at all... Maybe afm, knock sensor etc etc... Or maybe all in your head... Get a reliable boost gauge on it ASAP
Oh and yes legacy a are known to temperamental... Bit a lot of the time it can be down to vacuum/ boost piping being fitted the wrong way