2000 Subaru Impreza RS problem

Problem quickly stated: 2000 Impreza RS hesitates noticeably and feels like fuel is not going to the engine when you accelerate, particularly shifting into 1st or 2nd gear. Car ‘bucks’. Engine warning light not coming on, and when hooked up to diagnostic computer, no codes are coming on. O2 sensor replaced one year ago. We tried replacing fuel pump … no help. We then replaced MAP sensor at the suggestion of a highly-respected local mechanic, who said he thought the problem was definitely a ‘timing’ problem, that proper fuel mix was not being delivered to engine. MAP sensor replacement did not help. It’s getting expensive replacing things that don’t seem to help. Another mechanic friend told me that he’s quite sure it’s NOT the computer, as they never seem to fail. He thinks it might be the Coolant Temperature Sensor, but my son (a beginning mechanic) says the computer read out is not showing any trouble codes for the various sensors, and is reluctant and sceptical to spend the money to replace this sensor. Does anyone think they know what the problem is? Thanks!