need help with 97' 3.5 concorde, misfire

I have a 97 concorde with a 3.5, automatic, 225,000kms. The engine light came on 3 months ago stating the oxygen sensor bank 1 was sending no info. A few weeks later, it reported a misfire in cyl #5. I had the oxy sen changed by a garage. They changed the one on the drivers side by the exhaust( which side is bank 1)?. I also had the plugs, wires and coil pack changed but the engine still idled rough and the check engine light came back on within 30kms. Now, only 2 days after getting those repairs done, the computer says to check oxy sensor bank 1 and the engine runs with a putting out of the tailpipe, as though a sticking valve. It lost most of it's power and runs poor with popping noises, could this be the oxygen sensor, or am I looking for a new car? Any help would be appreciated, please help before I spend a fortune at the garage again!

your manifold gasket around the #5 cylinder is gone, a very common problem to occur on the 3.5 V6 engine.

would the intake manifold leak cause a sort of poofing backfire through the exhaust? It does seem to do it roughly in sequence to one cylinder. How would I check for a leak, carb cleaner spray or quick start around the intake manifold? Is bank 1 the drivers side. and could an oxygen sensor cause this problem.

disagee if its popping its a injector

Intake Gaskets,there is test.Spray brake cleaner or carb cleaner right at the intake gasket sealing area.If you see a change,intake gaskets.Most auto parts stores stock the intake gaskets.Watch out for this,one or two of the intake manifold bolts will break off.I have seen this happen and an easy out has to be used get the remaining part of the broken off bolt out.

Your car has 4 Oxygen sensors on it because of it being an OBD II car, bank 1 is actually the Passenger side of the engine so your mechanic probably changed the wrong sensor which still leaves you 2 to choose from.

The popping can still be from a Manifold Gasket failure if the failure is bad enough.