There is a bug in original SUNOS 5.5.1 and 5.6 kernel; If the CPU clock speed is higher than 429MHz, the internal tick calculation overflows, a smaller interval is set as system tick; It will cause premature wakeup from sleep and other side effect. The bug affects SSP4U, since the SSP4U speed is set to higher than 500MHz.
SUN provided patches for the problem:
SUNOS 5.5.1: 103640-21
SUNOS 2.6: 105181-06