We used to clean old well used CDROM lasers with a bit of spittle and a cotton bud on the laser eye.
Worth a go.
But in my experience with DVDRW+, it can always be a bit hit and miss.
Have you tried burning the disc at a low a speed as possible?
I Found RW- to be better, but I agree the full format should really have done the trick.
I did eventually give up though and bought a DivX player and used a USB key.
Now just the Xbox does everything.