Deteriorating burn quality

Hi, I’m fairly new to these forums and have learned a great deal. Thanks! I have noticed discs play well right after they are burned but then can’t be read months later (no major surface damage). I learned about cd speed and ran the quality test only to find out they were all terrible. I have Liteon shm-165p6s. I bought TY YUDEN000T02 discs and burned several discs. CD Speed read them as very high quality. I then discovered there is a firmware upgrade which I installed without any problems. Then when I re-ran cd-speed on the same discs, they read as terrible quality. Oddly, they play fine. When I burn new discs they read as high quality and 2 weeks later still read as high quality. Only the discs burned before the firmware upgrade read as poor quality. Does any one have any thoughts about this?