Quoted from Bax1:so if I am reading the schematics correctly, the +12 is the reset circuit? some caps and diodes in there and also tied to u51? could any of this cause the drop in +12v?
You can pull the 12v connector pin off the MPU and see where the voltage reads but I doubt the reset circuit of the MPU is causing the 12v to drop out. Its probably the large 5v load of the MPU making the 12v dip.
How are the power supply board connectors and fuse clips? Can look for excessive voltage drop happening across fuse clips, connectors, bridge, etc. Ground input plug 3J2 on PS has been known to burn up.