i have a 96 classic wrx with td05 turbo... few other bits.. pulls like an absolute train through 1 to 3.. in high revs in 4th and 5th it spools up but cuts in and out and hesitates... it flicks between 0.5 bar and 1 bar.. its jumps between the 2... any ideas?

over fuelling? air leak??

 

Is the car de-catted it sounds like your over boosting and the symtoms u describe are boost cut, This is usually more apparent in higher gears as there is more load on the engine,