North American Motoring - car can't handle accelerating on the highway

So a few months ago, I was accelerating onto the highway after picking up my car from the dealership after getting the steering rack replaced and things started going wrong. I have a 2008 mcs. The car sort of chugged and felt like something was misfiring and large puffs of black smoke were coming from the exhaust. I took my foot of the accelerator and the problem stopped, but as soon as I stepped back onto it it was chugging/hesitating and jerking again. I went back to the dealership and asked them to check it out and fix the problem so I could go back to having a smoothly accelerating car. Later that day they call to tell me they can't reproduce the problem and I need to go on a test drive with a tech. I didn't end up being able to go for a test drive because when I got to the dealership, all but 2 people were gone and they said they were closing. Awesome. So they tell me to just bring the car back if a light comes on because they can't reproduce the problem. In the meantime before I get a check engine light, my engine OVERHEATS for the second time in 5 months! I bring in the car to have them fix that issue and mention again that it's still having trouble accelerating. Very surprisingly I got a call the next day to say the overheating issue had been fixed, they replaced my water pump and I could come pick it up, no mention of the other problem. Finally this past weekend, I get the check engine light I was hoping for and whattaya know, cylinder 4 misfire comes up as the code. Dropped the car off, got a rental because they were out of their own loaners, and the next day I get a call saying they've cleaned the injectors and the car is running great. Woohoo! Except when I got the car, 10 minutes later I had the red symbol for "low oil pressure" come on repeatedly while I was sitting in traffic. :sad: Not the first time I had seen the symbol so I figured maybe the oil was low which is strange because when I had checked it less than 50 miles ago, it was perfect. Pulled out my dipstick, nothings on it. I even had someone else check it to make sure I wasn't doing something wrong somehow. Nothing. :mad: So I poured in a quart of oil and after a quick chugging ride down the highway it was very evident that the dealership did nothing other than turn off my check engine light and somehow leave me with no oil. This isn't even the first problem I've had at the dealership, but I would definitely say it's the first time I've picked up my car in worse condition than when I dropped it off.

Has anyone ever had the same chugging/misfiring symptoms and actually gotten them fixed???