This is no simple problem. I'd like a real solution, please.

Ok this is a long story but it will describe EVERYTHING.  Let's see how  
smart you are.  My car is a 2001 Kia Sephia 5 speed manual fuel injected.  
I bought it used from a friend.  The check engine light had been on when I
got it but it ran smooth and fast.  Throughout the next few months, I felt a
slight "hiccup" when I accelerated here and there.  It wasn't a big deal yet I
didn't feel right about it.  Soon after about a month or so, it started
hesitating very noticeably when I would be cruising (gas pedal held steady).  
Then a bit after that, the hesitating got more and more frequent.  Then the
hesitating turned into bouts of heavy running, loud, and slow until it all of a
sudden the condition seemed to "switch off" and run normal on it's own
from time to time.  I would usually rev the engine high to "kick" it back into
running normal.  I would keep it revved high to keep it going ok, but driving
slower brought back the hesitation.  But this would only progress the next
few weeks until it would simply do nothing but run loud, heavy, and shake
the car when idle...almost shutting off when pushing the gas pedal.  When
it would "switch" back to running normal,  revving it high wouldn't kick it
back to normal anymore.  Now when I start it, it runs heavy, shaky, loud,
and will no longer switch back to normal through high revving...it just
decides to STAY crazy now.  Mind you during this ENTIRE time from A to
B, I replaced the exhaust system, replaced the spark plugs, replaced the
fuel filter, replaced the fuel pump, replaced the fuel pressure regulator,
replaced the air pressure valve, replaced the air idle control, and
CONTINUING as all of these were an attempt to correct this most
frustrating problem.  All of these parts did nothing.  Every time I replaced a
part thinking this would be the solution...the car would never change as
now I can't drive it anymore as thought it still starts and runs, it has no
power to accelerate as the engine shakes and runs loud.  I'm ready to pour
gas on this thing and set it on fire and urinate on it.  I don't want to hear
about "take it to the mechanic" or anything or "get it checked".  Give me the
money and I'll do that right away.  Until the money tree grows in my back
yard, I want to hear a solution only.  The only things I haven't done YET is
replace the injectors, the injector wire harness, or the coils (though I did
replace the coils and it still hesitated and I returned them).BUT when my
alternator belt snapped and I drove it home on a wing and a prayer, it
seemed the problem went away completely and ran perfect...until I put a
new alternator belt on.  Now let's hear a good answer please.  You have to
understand that I'm not rich and I cannot go to a mechanic everyday...I
already have, right?  My patience is VERY thin because of this frustration
as this is the only car I can afford to have for a LONG time.  Kia specialists
preferred to answer this one unless you KNOW the answer.  Thank you
though and I do not mean to be short right now.