Poor idle and mileage

I have a 1994 Ford F150, 170,000 miles, 302 motor, 4x4, manual tranny. Recently installed Flowtech headers, after install, noted idle would bounce between 300 RPM and 1,000 RPM - nearly stalling out, when sitting still. Worsened when running defrost and engine not at operating temp. Still does it at other times, not as frequently. Replaced IAC valve, cleaned port, checked EGR valve and relpaced EGR/manifold gasket. Still does it, not as frequently. Also noted milage drop by 4 mpg. Runs great other than that, no check engine lights have been tripped yet, any suggestions?