MY POOR GALAXY

Good evening all,This is my first post where do i start I am the owner of a ford galaxy 2001 mark 2 manual 2.3 petrol car(ive owned the car 3 years) nobody can find the problem. Recently i have had a front section exhaust changed (which may be coinsidence)when fitted the car lost power on a couple of occasions while driving and when you put alot of revs on it sounded like it could be blowing but not all the time but checked out by another mechanic he said it was ok. Then the car seemed ok somedays then on a few mornings not one after the other but a few times in a 3 week period the car took a few times to turn over but eventually did.Also the blowing affect under bonnet (exhaust which i thought) happens after car revs drooping off sounds like a rattle but not all the time. Then i tried to start the car one morning doors open ok, dashboard lights on but no turn over at all i then jumped the car off my wifes no problem.Took to quickfit who said new battery, bought the battery ran ok for 24 hours (2 starts) then nothing again just dash lights no turn over. quickfit again looked. battery ok ,starter motor ok ,alternator ok even spoke to immobiliser technician who said that was ok because as i tried to turn car over (with no success)but red light on door stayed constant and not flashing but car would not start or even turn over. Then as about to call the aa tried the car which wouldnt jump off 10 minutes earlier and it fires up like there was never a problem the noise( exhaust like sounding still there).Drive home stop the car remove key tried to start again 2 minutes later and nothing just dash lights no turn over no sound oops nearly forget had the car plugged in to engine management wouldnt connect till car started up and when it did it said no error codes. aaaaaaaargh Not knowing is even worse please if anyone has any thoughts your help is much apprechiated regards my poorly ford galaxy