Knowing a full set of symptoms followed by applying theory and process of elimination in your head is the first angle of attack that should be used. When one does not know all of the vehicles symptoms, it makes a diagnosis very difficult, or a "shot in the dark". The OP does have some automobile knowledge, thankfully. Some, however, do not, and they become easily confused and start randomly changing parts, only to be disappointed in the end, and have a thinner pocketbook, with a car that still is not fixed. That's what I hate seeing happen to people.
White 2014 GT coupe / Auto Trans / Louisiana