I noticed that after a while the following message was reported:

    iy0 len = 310, avail = 260, setting OACTIVE

With IYDEBUG defined in if_iy.c, I see the following:

iystart called
iy0: trying to write another packet to the hardware
iy0: length is 66
iy0: avail is 7786
iy0: new last = 0x6196, end = 0x61e0
iy0: old start = 0x5800, end = 0x6196, last = 0x6058
iy0: setting 0x605e to 0x8136
iy0: giving RESUME_XCMD
iystart called
iy0: trying to write another packet to the hardware
iy0: length is 42
iy0: avail is 7712
iy0: new last = 0x61e0, end = 0x6228
iy0: old start = 0x5800, end = 0x61e0, last = 0x6196
iy0: setting 0x619c to 0x8042
iy0: giving RESUME_XCMD

and so on.  My interpretation of this is that packets are being
buffered but never written.