The drives in question are an LG16x CD-r (8016) which is the master and a Pioneer DVR105 DVD-r burner which is the slave.
PC spec is XP2600 processor, ASUS motherboard 512mb RAM running Windows XP.

Last night, I left the machine running as I had a couple of torrents and a couple of downloads running. Came to it this morning and the DVD drive felt very warm, even through the front plastic. The side is currently off the PC as I'd just fitted a new graphics card, and I felt the back of the DVD drive and that was very warm-almost too hot to touch for more than a few seconds. The drive was not responding when I put in a disc (either CD or DVD), so I shut down, and left the drawer open to allow maximum cooling.

After a good while, I re booted tried running a DVD and it played
fine-also tried a CD in the drive-it too played fine. So I tried to write a CD in the DVD drive..Using nero 6, what normally happens is the buffer fills within a couple of seconds, and the burn starts immediately after..now it takes the buffer over a minute to fill, and the burn rate was painfully slow, with the buffer emptying, then refilling over and over rather than staying full all the time . The burn completed after about 30 minutes, but the quality was awful, with crackles and pops all the way through.

I then tried to do the same recording using my CD-writer in the same machine (Both are on the same IDE cable) and that also did exactly the same thing.

I've checked the cables are connected correctly, they are, I've even
uninstalled both of the drives and Windows immediately picked them up again on re-boot.

I've made no changes at all to the configuration of the PC since yesterday when I made at least a dozen perfect discs. I've beem burning for 4 years now, and have never come across this problem.