1998 nissan altima PO 300

well there could still be a number of things causing that. you could have a vacuum leak some where causeing the cylinders to lean out. listen for any sucking noise and if you cant hear anything try spray some starter fluid around intake seal and suck to see if the engines rpms increas at all.

I didn't think that it was a vacuum leak because it will run fine for awhile (long enough for the check engine light to cycle off) then start missing again.

I had a similar issue with a 94 altima. I narrowed it down to one cylinder by taking off plug wires until I found it, and changed the injector on that cylinder. It fixed it immediately, and a week later the next cylinder started. It was the injector also. It ran great for the next few months until I traded it in.