03IronPony

I have a 03 Gt 5 speed. My problem is when i am running at anywhere from 45 to 55 in 5th gear it seems as though the engine is missing or cutting out or even some slipping in the tranny. When i let off the accelerator there is a ton of backlash as well. I took the car to a local dealership and they told me that its was driving malfunction basically the car is not made to be driven in 5 th gear unless your at 55+ MPH. Any opions out there to what or if i should do anything more. I have done hours of work to figure out whats going on with no outcome. Thanks for any help...