Alright, this seems to be the solution.
I booted the machine after I had come from work, tested a little bit with Nero and Instant Copy, then had the PlexTools do their work again. The first burn with PT worked fine, as it had been the case for several weeks. The second one failed, again I had to power off the computer. I downgraded the firmware to v1.05, and now the problem seems to be gone. Three consecutive burns with the same or different .wav files, and there has been no problem since.
For those who are interested: It is an ASUS P4C800-E Deluxe board with 1 GB of dual-channel RAM, an Intel chipset, an ASUS graphics adapter (V9999 Gamer's Edition), an ide and an sata hard drive and the Toshiba dvd-rom as the second optical one. For whatever reason I cannot plug in the Plex as the master, but this has not been any problem yet. The only old component is an Adaptec 2940 for nostalgy reasons and the scanner :-). All drivers and aspi components have been updated, all patches are set up.