I had some PNY discs that were rated for either 32x or 40x...I can't remember which. When burned on my old 48125W, it would go up to 48x. However, no matter what the speed I used, even 8x, I got unreadable blocks on the CD.
Like you, I went to 100% Fuji/TY discs!!
Mine were not Riteks...I think they were CMC...which explains everything!
My Plextor PX-504A drive (actually an NEC1100) burned them perfectly at 16x, though. Gotta love Plextors!
Soon to be proud user of Mandrake 10 (again!). SuSE 9 sucks!