Plex 708A's burn quality decreases with increased unit heat?

Hi everybody,
last night I was burning DVD+RWs and DVD+Rs and I was wondering about the following. The Plex 708A tends to get quite warm, slightly hot during operation. At first I thought about my HDD sitting 2 slots below, but it seems that the unit itself heats up.

As I checked the burn quality of the DVD+RWs I seemed to notice the following rule: burn quality seems to decrease when the unit is warm after a couple of burns.

I am not quite sure about this and would like to know if anybody of you has made similar observations? So far, heat in my units have not affected burn quality (in a CD-R-World).