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05 volvo s40 hesitating or losing power?

the s40 has been taken to the dealer and they do not know what is wrong. no codes. no check engine light. i had the transmission flushed, software update, injectors flushed, new spark plugs, turbo control valve changed. problem still exists! now this ‘loss of power’ happens more when accelerating from a stop/near stop but it also occurs when traveling at a constant speed. any ideas?