Unable to find cause of slow tape drive

I'll post the configuration specifics below. Here is the hardware

Sun Fire X4200 M2
SunOS 5.10 Generic_127128-11 i86pc i386 i86pc Solaris
Exabyte Magnum 224 LTO-3 Library (Ultrium-3 Full Height LTO-3 drive)

I was getting about 50 MBps write speed to this drive. Then without cause it
had dropped to 5 KBps write speed.
I have replaced the drive, the SCSI cables, and the SCSI terminator. I have
verified firmware is up to date. I have tested both with my backup application
and also with OS tools (mt, dd, tar). I have no errors or any indication in
system logs as to what the problem may be.

The only thing I can think of is that perhaps a patch or something else broke
a driver. I can plainly see that the tape drive has a high %busy. I'm not sure
what to make of the svc_t. I don't have any statistics from when things were
running correctly.

device r/s w/s kr/s kw/s wait actv svc_t %w %b
st12 0.0 89.9 0.0 5663.7 0.0 1.0 10.8 0 97

At at 64k block size that comes in the neighborhood of the 4-6 KBps speeds
that my backups are writing at.

I am using these drivers
pci1022,7458, instance #1 (driver name: pci_pci)
pci1000,5110, instance #0 (driver name: mpt)
st, instance #12 (driver name: st)
sgen, instance #20 (driver name: sgen)

The tape library is the only device attached to this adapter.

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