|You seem to have done everything I might have suggested. About the only thing left is the DVD/CD drive itself. The early drives had two lenses but modern ones have only one. You could try a lens cleaning kit for CD drives but it might be the drive electronics, which have to react differently to CD's and DVD's.|
Formatting might well be implicated but of-course CD-R and CD+R are not formatted before use like CD-RW and CD+RW. It would be "interesting" to know how it gets on with re-writeables, although I'm not sure what it would prove.
DVD/CD drives are not usually fussy about the "- or +" these days, despite them being pushed out by two different groups of companies. Worth trying a CD+R though, if you can get your hands on one, just to see if it does any better.
Keep watching tho, maybe someone else will come up with some inspiration.
Always pop back and let us know the outcome - thanks