check engine light blinking and loss of power.

What is your question?

My 1999 Infiniti Q45t was sitting in a car lot for 3 years. i got it running again put tires battery brakes and flushed fluids and all that. the check engine light has always been on due to an exhaust leak. it has started to lose power and the check engine light is now blinking everytime i drive it. i looked it up in the hand book and it means a misfire. the engine sputters and rpms drop, but it gets going again. i am running premium fuel through it and it is still doing it. i had the plugs changed about 4 months ago. being an infiniti they were hard to do, my mechanic told me. could one of them be out of place? and also when i'm under the hood i can hear the sound of air escaping from somewhere. seems to be at the back towards the firewall and cab. i can't diagnose that. could you please read my question and answer it to the best of your capabalities. thanks, Corey.