Running lean, out of ideas...

Hey guys and gals, I have a '97 Civic EX coupe. I have an AEM intake and OBX B5 catback exhaust. I haven't put much money into the ride yet cuz I haven't had it long. Since about the time I put the intake on, my check engine light has been coming on. I had some bad holes in my exhaust at about the same time, so I thought it might be that. After I got the new exhaust put on, the car did run better, idle was where it should be rather than almost at stalling point like it was before. The check engine light did stay off for much longer, but did eventually come back on. I cleaned my filter with the K&N kit, and ran a bottle of fuel treatment fluid through. My connection at the Honda dealer has confirmed that the light is on because it's running lean, but couldn't tell much more than that without doing a more thorough check of the car. My thought was that either my intake was too dirty, or that my injectors were dirty. I reset the ECU the same day I cleaned the filter, and put the fuel system fluid through. Before I could empty the tank, the light is back on. I'm going to reset it again at the end of this tank of gas. I don't think it will matter. It's pretty obvious it's something else. Spark plugs? Fuel pressure? ANY IDEAS!?!?! Thanks in advance. I want to get this fixed before I do any more mods to it.