Thank you both for your replies.
@allright: I agree about the 400 burns being "nothing". I have a 106 in another machine that is still going strong after almost 2 years. I'm a fanatic for keeping my boxes as dust-free, and as cool as possible. I learned about this the hard way.
@_chef_: I'm glad to hear I'm not the only one who has experienced these peculiar symptoms. I'm suspecting a dodgy batch of media. It doesn't happen on all discs, it doesn't happen to the same disc upon reinsertion. At the moment it is more of a nuisance, as when all is said and done, I do ultimately end up with a decent quality burn at the speed I prefer to burn at. When the current batch runs out, I'll endeavor to try something different, and will update the thread.
Thank you both, again.