Yeah, a one-buck part and an incredible number of man hours to figure out the problem. Somewhere there is an Italian electrical engineer who made my s--t list! I mean, seriously, Alfa sold these cars for how many years and couldn't fix this?
I'd been working on this quite a while, actually, swapping parts, checking voltages, checking grounds, checking current flows to ground, and just generally trying to figure out what was going on. Once I had enough parts stockpiled, I was able via swapping to nail down that the ARC computer didn't make a lick of difference but the display did.
After that, it took my buddy and I a couple of hours to figure out what the board was supposed to do. It's a pretty simple circuit, but we had a hell of a time figuring out the details for the driver chips so it took a while - just couldn't find them anywhere. Eventually we found a similar VFD driver chip with similar specs.
Once I knew how the circuit was supposed to work I spent an afternoon just looking at voltages on the board as various things happened, and even with a DVM I could see the voltage get noisy as things turned on and off. Using the freeze spray to further isolate the problem helped a lot. Hell, if I'd started with the freeze spray I probably would've figured the whole thing out a lot faster.
I actually bought an HP 1631D logic analyzer and oscilloscope to check for electrical noise and read the digital data stream but ended up not needing it. No big - been looking for an excuse to buy an oscilloscope for a long time.
So now I have a working ARC and a cool oscilloscope. Life is good