Can't record onto disc (by 2 users)

This machine was wonderful initially (bought 5/1/08) but now it is failing to dub much of the time when I try to dub to DVD-R. I was using Verbatim DVD-Rs but Sony DVD-Rs give the same result.
When it fails, it first makes a terrible noise before the error message comes up. I do a lot of dubbing, as I am trying to transfer all my VHS recordings.
I have tried optimising the HDD and dusting off and resting it, which appear to make it better for a while (though I am not sure which works) then it fails again. Now it is failing to dub to DVD-R all the time and will only dub to DVD-RW.
Could it be something about, or a fault in, the mode of recording setting? This is supposed to be automatic, but is the only explanation I can think of.
DVD-RWs work fine and make no noise at all but are more expensive than DVD-R, so I don't want to be stuck with them! Please help