The system crashed again last night and this is what I end up with by running a backtrace in ddb after a Stop-A:
cpu_Debugger(0xf2619d60, 0x38, 0x2e608f, 0xf03b1d30, 0x90001fa5, 0x8f) at netbsd:zsc_intr_hard+0x140 zsc_inter_hard(0xf2619d20, 0x1, 0xa33206e, 0xa56, 0xffff, 0xf0374800 at netbsd:zshard+0x3c zshard(0x0, 0xf0288940, 0xf00, 0x908010a7, 0xffff, 0x8c) at netbsd:sparc_interrupt44c+0x19c