Kickstart ROM for A1200 is NOT the same as A4000.
Main differences:
scsi.device, battery clock support for A4000, ZORRO support (Super Buster) for A4000, onboard FAST RAM support for A4000.
And few more.
If A4000 crashes with A4000 ROM and working with A1200 ROM, it means that you have problems with stuff not presebt in A1200: FAST, RTC, Super Buster. If your Super Buster is socketed, please remove socket and solder Super Buster directly to the PCB. This socket tends to loose contact with the chip over the time.
