Loss of power

car runs ok (ish) considering the b.h.p. it has but when approaching an incline in say, 4th gear ,( 40 or 50 mph) there is a noticable loss of power or torque if you like, resulting in a swift change down to 3rd, sometimes 2nd to get up the incline . Once over that it picks up again no probs.have done the usual fuel line cleaning, new oil and filters etc but still the problem remains. Could it be the cat.or dpf? Have been advised it could be a compression problem! anyone have any other ideas or similar problems?