It turned out that all of those disks were fine, so I continued downloading the other disks in the series by minicom'ing to my ISP's shell account, ftping them to the remote disk, and zmodem'ing them to my local disk. I played nethack on another virtual terminal while this was working.
Well, I switched back about 5 minutes later to check on the progress and it was just crawling, like 4KB every few seconds. I moved the mouse to hit the 'cancel' button on the zmodem transfer window, and the transfer rate shot up just then. I thought "well, okay..." and went back to nethack.
A few minutes later same thing happened. Move mouse, transfer speed goes up. I didn't understand IRQs at the time but I grasped apparent causality. I decided I'd try moving the modem to a different IRQ but that required a reboot. I wanted to finish the current disk set, so I sat there with a book in one hand, twirling the mouse in little circles with the other.
That's my hand-crank modem story.
To this day, whenever the network is slow, I twirl the mouse in little circles subconsciously.
 like these ( http://az413224.vo.msecnd.net/img/40934/m_40934_1.jpg ) not these ( http://images.yourdictionary.com/images/computer/_ACUPLER.GI... ).