The slowest speed doesn't always result in a better burn, so you should try burning at 16x and 24x to test them out. Did you leave Verify enabled? That's what tells you if the disc was a good burn or not.
As for CD-R media, Taiyo Yuden is the best you can get, and you can usually find it in online shops. A good quality new burner could help too.
Now, on another note, you do know that you can play PS1 games on a PS2 console and even on your computer, right? At least in my opinion, it isn't worth all the trouble for the PS1's finicky reader drive.