Lack of Power and hissing

I have a 1995 Pontiac Grand Am SE at over 150000 miles. About two weeks ago I noticed the car lacked some power when I pressed on the gas pedal. It seemed to get worse each day. It would be hard to accelerate after stopping at a light or climbing a ramp to the freeway. Then I began hearing a hissing sound. The hissing gets louder as the car was about to switch gears.. When it went to the next gear the hissing would go down until the next time the car was reving up to switch to the next gear. We first thought tha that it was the spark plugs and changed them and the fuel filter. But it just seems like it got worse... the last couple of days when i started the car.. the first 30 seconds or so the car does not want to move when the pedal is pressed.. then the engine seems to want to die. then the that problem solves its self and the good ol' hissing starts again.