Welcome:). The media you are using is good stuff, so that should not be a problem. Without looking at the kprobe graphs themselves, just the numbers, I would say those aren't really bad burns. Attaching disc quality scans to your post of the offending discs will help us help you. I suspect that the 50+MAX for PI and PIFs in this case may be just a single spike, since the averages and totals are quite good. So you are close to good burns. You should be able to burn at 4x and get good, maybe better results than at 2.4X burn speed.
One thing you might try is to use the EEPROM utility to 'reset learnt media' after a particularly bad burn. That will allow your burner to 'relearn' how to burn this media, or perhaps better said, this will allow your burner to 'forget' the bad burns.
Another thing you may try is some modded firmware like CG5G . This will void your warranty for the burner, but most people have gotten very good results.
Defragging your HDD before a rip and burn may also help, though it sound like you should ahve plenty of free space.
So, in summary, I would suggest resetting your eeprom, defragging then rebooting and try a few burns at 4x and post the scans(scanned at 4x scan speed, the forum standard for comparison). It may take 3 or 4 burns to get good results. If this fails to get good results, I would suggest, if you are ready to make the leap into the 'void' , to flash to CG5G.