Changing the fuel pump with it's mesh filter (it'was really clogged) and external fuel filter gave no result... Engine is still works as it has misfires while accelerating. I've checked fuel injectors for leakage or being clogged but they flow is equal.
I've just think maybe pressure regulator is faulty and has some leakage...

PS: Today I've found out from vehicle's owner that this faulty work began after he drove very fast (up to 180km/h (if it's possible)) on the 4th (not 5th) gear. But even this fact gives me no idea...