This is not true for alpha systems that were either resold or created by other companies. The AlphaPC product line was resold as beige box computers, and other manufactures made clones of the motherboards. The NetBSD/alpha page has a pretty decent look at the types. Your statement about them being complete systems is true for any DEC manufactured system that actually carries the DEC name (which AlphaPCs didn't really).
Hmm. My impression is that Alpha systems were built much more as
complete systems (as were Vaxen) than as the grab-bag of components
that PCs tend be. I did a little work at getting the 83815 and 82557
ether drivers to run before my Alpha died, and eventually I realised
that SRM probably wouldn't be able to tftp boot through them.
I'd start by replacing the 82559 with a 2114x. Mine is a DE-500 but
you might be able to find Netgear FA310s (not 311s), which should work
too.
What's in your /alpha/conf file? Mine was:
bootfile=/alpha/9apccpu
ether0=type=2114x 100BASE-TXFD
scsi0=type=ata
What's your /lib/ndb entry for the alpha? Mine was:
ip= 216.240.55.174 sys=alpha sys=¦Á dom=alpha.collyer.net
ether=0000f81fb442 # de-500ba (21143); original ether card
bootf=/alpha/bootalphapc
It might be revealing to install and boot a BSD or Linux and see if it
runs correctly.