1999 Ford KN Laser No Start Condition with a twist - HELP

[GrahamKNlaser]

As per title I have a 99 Ford Laser with an intermittent no start condition and I'm stumped.

Car will crank all day but not fire. The car has spark, and plenty of fuel getting to the rail, but won't go on it's own. There is also constant 12V power to the injectors when key set to 'ON'; however, no grounding signal from ECU while cranking (screwdriver and test light methods).

The Cam and Crank Sensors have both been replaced in the past 12 months.

The twist: The car will fire if starter fluid sprayed into the throttle body. It will also continue to idle on its own (i.e. not continually spraying starter fluid) after it has been started this way. I've had it idle for 10 mins without it cutting out (turned it off after this out of boredom). Also, once started, injector pulse is present (again, screwdriver and test light methods).

I have gone through the fuses and relays. All seem OK and, given car will fire up and continue running after priming with starter spray, I'd assume issues with these might be ruled out?

I guess what the question here is: what could cause a loss of injector pulse which then disappears after the car is forced to start??

ECU failing is absolutely a thought; however, trying to rule out all other possibilities before going down that route. Maybe a sensor/component (except cam + crank that have been replaced), or possible grounding issue, that could also cause this kind of condition.

Thanks in advance!