You might need to use discs of higher quality.
For DVD-R discs, you should use Verbatim DVD-R (With Super AZO) discs.
For CD-ROM games, you should probably try to get some Sony CD-R discs.
Since you have a HD7 laser, your console might prefer DVD+R discs instead of DVD-R discs.
My SCPH-39006 console (With HD7 laser) can read both types of discs though.
And like Player1 has said, it might be that your modchip is starting to fail.
It might have also been your laser weakening.
Also, try updating your CD/DVD writer's firmware too.
SCPH-10000 S. MINOKAMO v1.01 (Defunct)
SCPH-10000 S. KISARAZU v1.00 (faulty)
SCPH-15000 S. KOHDA (With warranty seal!) :D
DESR-5100 S. EMCS