After running for a few hours, my ide hard disk, hda, "loses" its settings--the one I care about is dma=on. When I boot, dma is set, and later I can reenable it with hdparm, but it always eventually goes back to dma=off. I am getting very terse messages in the system message log "kernel: hda: DMA disabled".

This is a new problem since I switched from kernel 2.6.11.x to 2.6.13.x. My chipset is VIA. The kernel module for the chipset is compiled into the kernel, and the "use dma by default" option is also compiled in. Actually, unless the chipset module is compiled in, I can't get dma to turn on at all. I don't know why this would be relevent, but, the distro is slackware 10.2.

Any relevant information, or any advice about how to troubleshoot this is appreciated in advance!

This may happen if the drives is old/buggy, the motherboard too old for DMA (P2 motherboard and lower... linux usually bypass the BIOS to activate DMA on old hardware but it's not that great) or if your wife has a bad day ( read : anything, you are maybe just unlucky).
Usually, the best thing to do is to try to slowdown the drive. Use "hdparm -vi /dev/hdX" to find out what DMA settings your drive is using. There is usually several "DMA modes" supported, as example, if you are using, let's say, udma5, then it may be a good idea to slowdown to "udma1" or even lower (mdmaX). Take a look to hdparm, it should be clear enought

Nb 1 : if "hdparm -vi" can't tell you which DMA mode you are using, then there is probably a big bad problem there, mean the driver is trying to use a mode that your drive isn't even supporting, to slowdown is important then.

Nb2 : it's quite normal that you can't use IDE driver as module. Since it is the first thing loaded on boot, if you want to use this as module, you have to use the an "initrd" ram disk, as the kernel need to read the disk ( and so to have an IDE module loaded!) to load your module. In that case, the kernel just usu the "default" ide module, which is slow as hell and prevent you to load any further IDE drivers.

Thanks. I'll look at these, particularly the drive speed (udma) idea, which seems most likely in my case. Appreciate the help.

P.S I always dread the "mabye its just bad luck!" option.

If the dma drop occurs at regular intervals, you may want to see if a cron job or other scheduled task is restoring old settings or the like.