loss of power after 3500rpm
 

please help

ive got 2000 wrx power loss at 3500rpm then comes back in at 5000rpm been at subaru sheffield all week had diagnostic done no fault found

swapped maf, both oxy sensors, knock sensor and checked timing returned with same fault.

can anyone help before i fetch the petrol and matches?

dump valve?, any mods to the car?

no mods totally standard except for exhaust