"Do you know if there's any way I can increase some timeout, or have larger blocks DMA'd less often or something?"
There is a way to turn off dma for a particular device but I don't remember what it is. People use it sometimes when they have buggy IDE controllers that don't respond to DMA correctly. Search the forum for hard drive problems that are solved by turning DMA off and you can find out how to turn DMA off for a particular device.
"Do you suppose the 90KB/s speed is due to the errors, or would a 10Mbit card really be that slow?"
The speed might be limited by something other than the card. It could be line speed. It could be the speed of whatever is at the other end of the line. And yes it could be that the hardware is fooling around a lot recovering from non existant errors.
Be prepared. Create a LifeBoat CD.