Hey guys got a question. About a month or so ago my exhaust fell off from in front of the converter all the way back. It was super loud but drive it for a week or so cuz I had no choice. I drive an hour one way to work pretty much all highway. It started to sputter when on the gas no matter what speed. So I got a whole new exhaust system. It's a cheap one but it works took it to a mechanic an he drove it for 2 weeks said he couldn't find anything. Now after about two more weeks driving it with no problem it's doing it again but only at 70-80 mph and not as violent as before just a couple times. Any ideas.

Work done to it:
Pcv valve
Radiator
Exhaust
Speed sensor
Spark plugs and wires
Valve cover gasket ( not the cork one)
Air filter