Your post covered most of your machine except, power and cooling.
With that setup, did you have a 450Watt or better supply fitted? Is it cool and breezy inside?
Bob
I have a Plextor 708A that started giving me frequent burning errors. I had been using the same software (DVD X-Copy Platinum) and brand of media (Prodisc) for 6 months and never had any problems... but then the read/write speeds both started getting slower and I began getting write errors now and then. Eventually, the errors became so frequent that I rarely had a successful burn, and the read/write speeds slowed down drastically. It was taking over 2 hours to burn one 4x DVD. I tried new software (1ClickDVDCopy) and another brand of media (TDK) and the problems didn't let up. After upgrading the firmware and still having no luck I figured the drive just must be worn out since I had burned a couple hundred DVDs since I got that burner. So I bought a new burner, a Samsung TS-H552B... and the problems continued. Only now DVD X-Copy Platinum doesn't even recognize blank media (Prodisc and TDK), so I can't even attempt a burn with that software, and the 1ClickDVDCopy software gives me errors of this type everytime, however the read/write speeds seem ok. I updated the firmware, checked to make sure both brands of media are compatable, reinstalled the software, and even reformated.... nothing changes. I'm assuming it's some sort of hardware problem, but I don't know that the problem is in the drives since they're two different brands and one of them is brand new yet they both seem to have the same problem. Although the error report from 1ClickDVDCopy says "incompatable format", both brands of the DVD-Rs are confirmed compatable by the drive manufacturers.
My PC has:
Windows XP Pro
512MB RAM
Gigabyte 8IK1100 motherboard
P4 2.8Ghz CPU

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic