Don't over analyze things and look at the most simple things first.
One of our clubbie guys had a car that had been going like a train but suddenly wouldn't start, he was off to buy a new starter motor.
Turned out to be an HR joint on his battery isolation switch.
Same car started to stall when turning left even with half a tank or more, must need a new pump the lad said because it sounds funny.
Funny was because it was pumping bubbles, turned out to be an in tank prefilter being clogged by the gunk that RON 98 can produce.
A quick blast with the compressor and it was smiles all round once again.
In both cases the forum help had him almost building a new car.
Another mate couldn't get a 351 to run smoothly after new plugs and leads, more wild ideas were offered.
All he'd worked on in the last 5 years were Chev.
