Shifting Issues after Fresh Install?

I have a 2003 Silverado 1500 that I just had a fresh torque converter and transmission installed. Shortly after installation, I began noticing issues with shifting, and it seemed as if the transmission was slipping out of overdrive, but only when letting off the throttle. The problem persisted, and I was informed that the filter needed to be replaced. I did as advised, but I still have to give it a lot of gas in order to get the truck to shift gears. There was also a loud noise coming from underneath the vehicle. Could this be an error with the installation of the torque converter to the flexplate? Loose bolts or even misalignment during install?