Philips041 can be made with anybody withouth additional information (read serial numbers I can not confirm who's made them. )
I do have seen reports from other people pointing to ritek and cmc both.
Now about the quality what are the storage conditions.
I can think of 2 options what has happened to your disc's
Both error issues will not show up on Lite On's our Benq scans. I don't know about plextor drives since they have a few more test options which might help them. But still the change is small.
Profesional analyzers however can identify these problems.
Problem one the spiral groove has been deformed and the drive can no longer read it correctly.
Problem two phase/reflectivity problems.
Both are rare however the first one has been spotted on some early DVD-rewritables.
The second one only is possible with low temperatures and huge temperature shifts. I was able to recreate this problem with cd-rw's once.