We're running a 1000 hypertherm. It doesn't have the power that your 1650 does. I think that even with the extra power you have, 6mm material is to thick to cut with fine cut consumables. I'd have to use 60 amp consumables to cut 6mm, fine-cut wouldn't even be an option for us. Try slowing your feedrate down considerably, that might help you out. On the other hand, if you slow it down to much make sure you click your THC to manual so the torch doesn't dive into the material.
I'd also give the guys at Hypertherm a call, or look for Jim Colt on this website.
Hope that helps you,