Now I have more data.
My suspicion was true, it happens between the 10x to 12x speed shift!
If I record any disc at 12x the Spike point moves from 2.2G (when 16x is used) to 3.8G, because the 10x to 12x happens at that particular position!
After a few more tests, this time at 8x, I would expect no spike at all since no 10x to 12x were used. Well I was wrong, they did have a spike (at 3.4G) and I said to myself well what is happening now? And I recorded another disc once again at 8x, then using IMGBurn export function to DVDInfo, I saw a speed down (not due to HDD output fluctuation or the OS using the HDD) because it repeats itself anytime with any disc.
Well the problem isn't solved but at least I know that I should stay away from 8 speed or up, and avoiding speed drops during recordings. Go figure, I paid for a 16x recorder and now I can only use it at 4x or 6x speed :(!
Conclusions: My drive has problems when re-liking function is used. I will try to swap the unit and hope the new will be free from these annoying Spike problems.
I will post my scans soon.