Starting problems

The POWER switch is the cheapest and easiest item to replace. Should you find that the problem persists, the next question would be whether the problem can be replicated on demand. For example, do you find that when the weather is quite cold, the car may fail while in warmer weather the car works reliably. A loose solder connection would result in intermittent operation that may come and go as the temp changes.

If that is the case, then one way to narrow down the problem would be to cool down the suspect ECUs, one at a time, to see if the problem can be induced. There are chemical sprays that can do that - or put a Baggie full of ice on the part. Then use a hair dryer (or heat gun) to warm up the part, to see if the problem goes away.