HELP! I HAVE AN AIR CONDITIONING SYSTEM THAT I ABSOLUTLY HATE AND HAVE HATED SINCE I MOVED IN 10 years ago!

This stupid thing in mounted in the attic (were the heat built up is unbelievable) and it gets to the rooms by means of TUBES. Problem is I can barely feel the air moving and it doesn't cool a damn thing, even after having te outside unit replaced. HI VOLUME AC??? I think not! Any suggestions as to what the problem might be? If it is 79 degree's out side and sunny, it can't even get the temp under 74 degrees.

HELP!!!!!!!!!!!