Sputtering 5.3L V8

I have a 2006 Saab 97x. While the car is not very common, the GM 5.3L V8 in it, is. It runs perfectly fine under normal driving conditions but for the past several months it has been running like a Slaab instead of a Saab. It has been backfiring, sputtering and has had loss of power issues but only during hard acceleration.
When my check engine light came on at 50k, the dealership changed all 8 iridium plugs and when it happened again at 75k, only one was replaced. Aren’t these supposed to be 100k plugs? One of the exhaust manifolds has been replaced due to a leak and the other one has a leak now. Fortunately, all of the previous repairs have been covered by my warranty. Unfortunatly, my warranty has run out and the other manifold is no longer covered and seemingly neither is this problem. Most recently, my new mechanic used some fuel injector cleaner something or other to no avail.
I tow a 5000# camper and can’t have these issues when summer comes. Is it the plugs, manifold, engine, something else? Any help is greatly appreciated.

No Fair !

The Mechanics Got The Vehicle, Road Test /W Symptoms, And The DTCs (Diagnostic Trouble Codes).

All We Get Is A Description Of What’s Been Going On With This S[L]aab !

Any chance you can give us the codes from the 50,000 mile incident and any from other times, including now ?

CSA