a few things to try-
1. unplug the 3 connectors on the ssio, and see if you still get the SBIE.
2. can you read the markings on the roms on the ssio? Are they in the correct position?
3. Test the eproms and ram and cables.
4 i think you said you replaced the z80 already.
5. visually inspect the sockets for corrosion and missing pins.
6. Download the schematics from
www.klov.com and trace continuity of every wire coming off the 19 and 23 pin connectors on the ssio.
You've got more than one problem that i can see. Your weapons van lights shouldn't come on unless you've credited the game up. Its on the same connectors as the ab. pos. board, as is the Cheap squeak deluxe - it shouldn't play peter gunn unless you have started a game. It playing when the machine's powered on indicates its not recieving a signal from the ssio.
Why that is? Could be a bad connection, or something bad on the ssio. Being that it keeps giving SBIE's, there's most likely something going on with it. they're pretty much interchangable with other mcr ssio boards. That looks like the next step to me, getting rid of the sbie.
Oh, and the dac on the Cheap squeak deluxe sounds like its going out. I forget the part number, but its socketed on the CSD - i think electronic goldmine has them.