I had a DVD burn take 30 minutes once. It was only my 2nd burn with my 716A. I was very confused/upset until I figured out that my HD (and system) was overheating. I use a silentdrive enclosure which really keeps the heat in, but I can imagine a drive overheating in other cases, too. When hard drives overheat, they slow down a lot.
My drive buffer kept going from green to red and back, and at some point my RAM buffer was down to 27%, and the burn was basically a 4x burn, and it was crap :Z
I'm not saying your drive was overheating, but that was definitely the cause with my "30 minute mystery burn". In fact, I had a system overheat and BIOS-protected shutdown around that same time after my first burn while I was doing read tests. Quiet computing can lead to overheated systems when stressed, and I built my system for quiet over speed. I turned off the computer, turned up the fan on my Zalman CPU cooler, took out my trash, checked my mailbox, and came in and did another burn with no problems.
Reading/writing 4GB so quickly produces a lot of heat--all that hard drive activity and all.
And by the way, the Plextor PX-716A is pretty quiet--even quieter burning DVD's, I think, than CD's. Definitely not loud, even without riplock. I'm happy about that.