I'm not positive, but I think there were some revisions of the 16x plextor drives that were, how shall I say it, 'tempramental.'

I have the 12x Plextor and there are only 1 or 2 discs that have a problem when reading the outside tracks, which I always thought was some kind of ultraviolet damage, since it only ever happened to discs that were left in my car for a while. My extraction speed drops to around 0.1x at that point, with the drive spinning up and down, similar to the way you describe.

Perhaps it's just those "troublesome" discs. I would just use the Toshiba on those instead.