2000 Pontiac Bonneville Runs but barely

Ok had my car sitting for about 4 months went to start it one day nothing. Change bbattery and spark plugs wires did a oil change, car stared fine good to go. Fast forward three weeks cars shut off while driving try to restart but not successful till 15 mins later after numerous start attempts. Mechanic replaces air sensor, problem seems to be fixed($750 ny)./Now the car runs but very badly there is a loud rattle whenever its floored and it changes to a higher gear only when you let off, if you keep your foot there it will stay in 1st or 2nd the whole time rattling away, rattling stops when yopu let off and car will change to higher gear but still slow on the pick up. It sounds like there is a leak coming from the muffler (uneducated guess) and the car will pick up but very slowly. The voltmeter also surges up and down betwwen 12 to 15 volts only checked it on the car dash voltmeter didnt actually put one to the battery or alternator.I dont where to start attempting to repair this myself dont have the money to pay for miore repairs HELP ! I'm mechanically inclined but mostly on diesel generators but I follow instuctions well. PLEASE HELP