i have a 91 crx si w/b16a1

when i am driving in 3rd gear only, after 7k rpm, the car will not rev any more, it takes a few seconds so it can rev higher then 7k rpm in 3rd, is it my spark plugs or is it my psi, i have my aem fpr set at like 42psi, the car did not do this before, these are the mods i have:


b16a1, ported and polished head w/itr intake valves, itr springs and retainers, stock block, aem short ram intake, thermal catback ss exhaust, power core race cat, jg 4-2-1 header, itr trottte body, itr intake manifold, itr cams, skunk 2 cam gears, clutch net clutch and flywheel, walbro 255lph fuel pump, aem fpr, aem fuel rail, vtec controller, chipped pro mugan ecu to redline at 9300 rpm, vtec set at 6000 rpm