Strange Problem on DECstation 2100

I have been trying to get my 2100 working. I have been using MOP to boot the machine and try to install Ultrix, although this is not working for some other reason at the moment. Until this morning, it was booting from MOP just fine, but now it, after leaving the machine overnight and not doing anything to it, I get this when I try to MOP boot: Ultrixload - V4.5 Thu Sep 14 07:07:53 EDT 1995 Excptn: (vtr=NRML) Excptn pc: 0x807001f8 Creg: 0x30000010(CE=3,EXC=RADE) Sreg: 0x30000000(CU1,CU0,IPL=8) Vaddr: 0xaaaaaafe Sp: 0xa13fbba8 0xa13fbba8 : 0xaaaaaaaa 0x80701310 0xa0003b60 0x80700000 0xa13fbbb8 : 0xaaaaaaaa 0xaaaaaaaa 0xaaaaaaaa 0xaaaaaaaa 0xa13fbbc8 : 0xaaaaaaaa 0xaaaaaaaa 0xaaaaaaaa 0xaaaaaaaa 0xa13fbbd8 : 0xaaaaaaaa 0x80700048 0xa00039c8 0xaaaaaaaa 0xa13fbbe8 : 0xaaaaaaaa 0xaaaaaaaa 0xaaaaaaaa 0x00000000 0xa13fbbf8 : 0xaaaaaaaa 0xaaaaaaaa 0x00000001 0xa00039c8 exit(-1) The really strange thing is that if I attach a CD-ROM drive and boot off that first, then try the MOP boot after that, the MOP boot works! Before anyone asks, I have other problems which mean it won't successfully install Ultrix from the CD-ROM, and I am trying the MOP route because of this. The CD-ROM problem is something to do with the block size, I know about the block size jumper and have tried with the jumper installed and removed. I have a workaround which is to connect the CD-ROM, boot off the CD-ROM, let the CD-ROM installation fail, then unplug power from the CD-ROM and boot Any ideas? Regards Rob