Relay Box in a 1999 Mercury Sable

My car does not start every time I turn the key. The car has new ignition
coils, plugs, and plug wires. The battery and starter are good. I did a test to
see if power is reaching the spark plugs and it sometimes does and
sometimes it does not.
I thought it may be an issue with one of the relays but I cannot find a
diagram of the relays that are in a small box by the battery. I have all but
eliminated the relays located in the power distribution box. There are a
handful of relays located in this small box and no fuses.

Any advice on other potential causes of this will be greatly appreciated.