i wouldn't be the least bit surprised if it was the drive crapping out. the drive in my PC150 coastered out almost exactly a year after i bought it, hm, summer of 97. i don't think it was an nec drive, tho..
fwiw, it may be the extension from nec. installer cds, from apple in particular, are picky, especially when trying to boot from the cd..
could always go ask at powerwatch.com. someone there's probably seen this before..
and one extra mini-ellipse..