Welcome to the forum
Most of times this issue is due to a low quality disc.
What discs are you using exactly? Can you post here also mediacode? You can retrieve mediacode using cd-dvd speed (see the "Disc Info" tab). Too bad, TDK sells many different discs with the same brand
Moreover, slowing too much the burning speed can create a worst burning. For example, if you burn a 16x certified media @2x, there is a high probability to obtain a coaster.