When you are burning a disc, has no-one ever noticed (particularly at lower speeds of 2x and 4x) the recorder buffer, which usually hovers around 91-98%, falls at fairly regular intervals to 7%, then quickly recovers back to normal. On a 4x burn it happens as often as 7-9 times.
I've seen people blaming these brief drops in buffer level to multi-tasking but it happens even if all you are doing is burning a disc. At 12x, the drops still occur but are less frequent. I always thought it was something to do with the drive adjusting the spinning speed of the disc and always thought it was responsible for the PI spikes you are talking about.
I have 4 NEC burners and the brief buffer falls occur on all of them.
I used to have 4 Pioneer 105's and nothing like that ever happened on them...