2012 Z6 runs like it wants to quit between 5 to 15 mph,it just seems to bog down and if you give it just a little gas it acts as if it wants to shut down .But its OK at idle and any speed above 15 mph,you can punch it at these higher speeds and it will get up and go

Feels like you came out of a corner on 3rd gear? This is either dust packed in your clutches. The rollers cant move like they should. The helix is likely chewed up if it is or you need to adjust the secondary spring for more back shift. Think downshift and you'll get the idea. Mine is pretty stock. I'm at 25g. Rollers and clocked secondary at A1. I still get a little lag at 15mph but it  is much better than before. I don't need to stomp in it anymore.