Do you have a test unit to check for error codes, some units use a LED test
lamp you can make yourself or get a tech to make for a six-pack. I'm from
down under so we have holdens instead but most manuals give you the details.

That gives you something to start with. Check all the connectors for
corrosion as they can give problems. A lot of car ones are changed with no
real fault.

Do you have any spares to try against, or is one available from a wreckers?
Is the spark timing controlled by the same unit or is it correct? One of my
cars, a 1989 XF Ford Falcon had a failure on a level crossing one wet rainy
night when water flooded the vent and wet the spark control box on the
passenger foot wall. Removing the circuit board and flooding with a can of
cheap brand water dispersing spray and draining before putting it back
together and driving it home. Testing the unit was done with a paperclip
shorting the connector for distributor setting allowing the spark to bypass
the flooded box.

A good switch to mount on your dash may be a test mode for the distributor
to give you raw spark with no advance, and another to turn on the fuel pump
feeding the extra nozzle in the inlet manifold giving you enough fuel for
80% power to get you home when your ecu dies.

You could set this up on your test bench to help isolate your problem.

Hope this helps,
Peter