No one can figure out whats wrong with my car

3 mechanics, several backyard, myself and many others
CANNOT figure out whats wrong with my 2003 Forester.
Long story short. Ran perfect, threw a rod through the
motor. Bought a Japanese used 40 to 60 km motor. Had it
put in with a new clutch and spark plugs. Perfect. A
couple days after I got my Car back W/new motor it
started hesitating if I pushed it slightly. Then it seemed to
get progressively worse. Until when I floored it it wouldn't
even go. Just acted like it was loading up and back firing
slightly.  Then randomly it would all of a sudden run fine
(but not at top performance). Ok so thru code reader 420
$660 for a cat conv. So we just punched through it. Ran
fine for a couple miles and then started acting up again. I
changed the fuel filter then two weeks later changed it
again. Changed the o2 sensor since I got that code too.
Ran fine for literally for a Couple minutes. Still got o2
sensor code same bank- returned first one as faulty and
put in another one. Still same problem. My boss and
husband all along maintain it’s the in tank fuel pump. Not
fuel pump. A mechanic Tested it. Always get a 1518
starter switch code. Replaced started (not cause of code)
it needed one. Half the time if it starts running bad I will
turn it off and pop the clutch when it’s tolling and
sometimes it magically runs fine. Like resets itself. Idk. SO
my son said MAF sensor. So I bought cleaner. Cleaned
what I thought was it via YouTube. But I got the name
confused with something else. Anyway after that it seems
like someone’s taking my throttle cable and jerking it.
Totally new symptom. Along with old symptom
Cars been at a 5 star rated mechanic for 5 days and he's
having an issue figuring out what's wrong with it. Possibly
computer. What about the coil pack? No one's mentioned
that one yet. Only due to the backfiring (like a distributor )
I thought it mk ing but then again I can work on an old
Chevy 350 or a old VW bug, older motors are simple. This
has been ongoing for 18 months now.