most likely as was suggested before you are experiencing buffer underruns....
so here are some suggestions:
in alcohol there is a buffer bar check the bar and see what it shows. if it is repeatedly going to 0 this will cause the drive light to change color and suspend writing until the buffer is refilled.
another question/suggestion: if you are recording on-the-fly then stop and create and then burn images.
next suggestion is to try the software that came with the drive. this will determine if the problem rests with the alcohol software or the system setup.
also you have been asked repeatedly about the motherboard. this is because there are certain chipsets that will cause problems. please reply with your motherboard chipset or let us know if you need help to locate it.
in your first post you noted that 4x burning is faster than 8x. that will happen if the drive is repeatedly starting and stopping while the buffer is emptying and refilling. if you can give us more info we may be able to help you so you can get a steady stream of data to the drive so you can burn at 8x.
you also noted that the dvd burns 4% fast and then slows. this is because the drive is zoned to burn the first 4% at 4x then to 6x for a while and then an 8x zone. this again plays into a scenario that requires the info and testing on your part noted above.