All,

Peter's suggestion to double the client cache (20 --> 40,000 blocks) seems
to have turned the trick.  I'm able to get through a complete cycle of:

$ make mrproper
$ make xconfig
$ make dep
$ make boot
$ make modules

(on Linux 2.1.85 source tree) without mishap.  

Someone more knowledgeable about Coda internals may wish to investigate
why smaller caches cause the build to fail sporadically with truncated
files.

At any rate, I'm up and running...

Thanks to all who made suggestions!

Steve