I would stop changing things, an engine is still just an engine no matter how many fancy do-dads they add to it.
Computer, sensors and controls just make make it more reliable, probably not your point of view right now, the "good ol' days" of carbs and distributors sucked, you had to adjust stuff all the time to keep engine running right.
I think the main issue these days is that people don't have to mess with the engine, so when there is a problem they haven't been practicing one or twice a month, like we had to in "the good ol' days", lol.
I would invest in a $25 Vacuum gauge, and read up here on some test results: Technical Articles: Engine testing with a Vacuum Gauge - at Greg's Engine & Machine
Low vacuum could mean timing belt has slipped, or a vacuum leak
Slow vacuum return could mean clogged exhaust
How many miles on the engine?
Does it run better cold than warm or visa versa?
Does it run better at high or low RPMs?
Any gas engine is still ruled by the big 3
Fuel, in right mix with air
Spark, at the right time and intensity
Compression, above 120psi, 2.5l should be 150+ psi