Orange light on burner when DVD Spins down ...?

I have been experiencing some wierd Orange lights [aka buffer underruns/smartburn kicking in] since I replaced my NEC DV 5700A with an LTD-163D. I did a few burns siting in front of the PC listening to the drives, and crazy as it sounds the orange light is seen at the point the LTD-163D spins down. I checked on 3 seperate burns, with different media and each time as the DVD drive slows the underrun occurs. Yes, they share the same IDE channel and DMA is enabled, but this didn’t happen with the other DVD drive and shouldn’t happen with this one either.

Anyone got any clues?

LTR-40125S Firmware ZS0K and later ZS0N
LTD-163D Firmare GHR3