Is the game blowing the solenoid fuse when the coils all activate? Does this happen the moment you turn on the game?

My initial thought when you described the problem was possibly an issue with the blanking circuit, still thinking it might be. The blanking circuit should be low when the board first boots, but then go high quickly after. If it doesn't, all the coils may fire simultaneously upon boot-up. This may not be your problem, but worth checking. In my case, it was a matter of changing a 1uf tantalum cap, a resistor (can't remember value), and the 555 chip for the blanking circuit. I didn't narrow down the exact culprit (probably the 555 in hindsight)...i know you mentioned you have a good blanking signal, so this may not be of any help. I just figured I'd mention it because i spent a *long time* trying to diagnose a similar issue on my BK where it would intermittently blow the solenoid fuse (due to firing all the coils simultaneously), but only upon boot-up. Good luck!

Nice work! That's a cool mod, i will definitely add that to my BK's mpu next time it's out.