Sounds like a heating problem to me, BoB, with such a small number of errors at first, then none later in the night where it should be cooler :p You might want to consider opening the PC and cleaning out the cakes of dust that builds up on the CPU heatsink and sometimes around the RAM - it acts as insulation making cooling less efficient, so since it builds up slowly, it makes sense it's effect wouldn't have been noticable until recently.
As for torrent programs making music play slowly even with low CPU load, it's can be because you're downloading+uploading at a high speed, so there's a lot of hard disk activity due to a high number of random read/writes, getting in the way of Winamp reading the mp3 or whatever in a timely manner. This can be made worse if your drive's very fragmented. It can also happen if you're using a lot of the swap file, because a lot of memory is being used, more than the amount of RAM you have. I think Azureus has some options in it to ease the amount of disk activity it generates.
You could do what I do and tell winamp to buffer the entire file before playback into RAM if it's 32 MB or less, which works nicely for almost all my tracks :D
