In addition to suggestions upthread:
• Check all relevant connections, feed and earth to the battery, ECU, and engine earths. Inspect, wiggle, disconnect, clean, reconnect.
• Check battery voltage at rest, minimum during starting, and at 4,000 rpm.
• Check ECU fuse contact and connections. (And what's the fuse rating?)
• Check spark plug wells for water and spark plug leads for positive engagement. (But sounds like all cylinders losing power, not one.)
Jonathan