Is my fibergl ductwork lining improperly installed?

You've done your homework. Goodman is only responsible for the machine. The installer is at fault if all you say is true. There may also be some culpability in the company that sold you the machine if they were using sub-contractors for the installation.My guess without examining the situation is that the furnace is over-sized for the area your trying to heat and 6 inch duct is too small for the CFM the machine is putting out. I wish you the lord's blessings and good luck.