91 240sx KA24DE manual. The problem: Car will intermittently bog / shudder at random times, exhaust stinks like its running rich. I tried pulling over and pulling wires to see if its a misfire / which cylinder but its so damn intermittent that it returns to normal before I can get under the hood.

Background: This problem started out as happening only once a month then got more frequent. Eventually it got to the point where it was happening several times a day (about every ten minutes on average, but at random times. hot or cold doesn't make a difference). I changed the spark plugs (~0.040 gap) and the problem went away for about a month. Then it came back slowly. Now its getting more and more frequent...

So I figure its an ignition related problem based on the new plugs fixing the issue temporarily? I remember getting new wires a few years back but can't remember which 240 I put them in . Could this be a fuel related issue (injectors?) even though changing the plugs fixed the problem for a month? (I know cars can be weird like that)

I'm gonna check the ECU when I get some free time

Well it might be smelling rich just because you're misfiring and not actually burning the fuel.
Check your engine grounds and pull your plugs to see what they look like. If one of them is super black and sooty, you know you've got a problem there. Check your wires to make sure nothing is worn through and shorting out.