Well, the burner is over 5 years old, but if it was the burner wouldn't it fail to burn at all?
Isn't that what is happening? It can't get to the end of a disk any more, the laser diode which makes the beam which "burns" the disk is crapping out part way through the disks. If it is 5 years old, and you have done the other steps, ie changing disk brand, slower speed, I would definitely suspect the burner.
Maybe if you have other burning software on board such as Nero, you could give that a whirl, but repeated failed burns tell a certain kind of story...
I don't know what kind of PC you have, if it is a desktop or a laptop, but, believe me, burners are a lot cheaper than they were in 2002!! Like, desktop PC DVD writers are under $50 now. Nearer $40 in some cases.