1) Try turning off TAG QUEUEing.

this is only possible with DEBUG kernels?  i haven't build a DEBUG kernel
for this machine for a long time :-)
   
   2) ncr53c9x_select() checks for an ESP100 and turns off
   SELATN3.  You have an ESP100A which may have a similar
   problem.  Try adding it to the switch() statement.

i also have an ESP100A in my U1 that does work:

esp0 at dma0 slot 14 offset 0x8800000 vector 20 ipl 3: ESP100A, 40MHz, SCSI ID 7

i'll see if this makes a difference.
   
   3) Set ncr53c9x_debug to 0xffff and see what's really
   going on.

waiting for said DEBUG kernel...