2007 Chevrolet Tahoe Z71, miss firing

Hello, I purchased a used 2007 Tahoe Z71 and it's been miss firing (missing) 2 wks after my purchase. The check engine came on so I had it serviced: new axles, fuel system cleaning and another week later they had to replace the fuel pump. The techs say my spark plugs are good, but after all this my tahoe still misses while stopped. It does have 97k miles, use to be a fleet. Any idea? Thank you in advance.