misfire under load

my 99 tahoe misfires underload, mainly up long hills or mountains. replaced plugs,wires,cap,rotor and new fuel pump and filter. misfire was just in cylinder 4, replaced injector. Ran better for about a month but now it is misfiring in 4 and 5, ran out of ideas need help!!!!!!!!!!!!!!!!!!!

my 99 tahoe misfires underload, mainly up long hills or mountains. replaced plugs,wires,cap,rotor and new fuel pump and filter. misfire was just in cylinder 4, replaced injector. Ran better for about a month but now it is misfiring in 4 and 5, ran out of ideas need help!!!!!!!!!!!!!!!!!!!

DO YOU HAVE A SERVICE ENGINE LIGHT?IF YOU DO, CHECK OR GET YOUR CODES CHECKED. YOU MAY JUST NEED TO CLEAN OUT YOU THROTTLE BODY& EGR VALVE.OR CHECK VACUUM LINES. WHEN WAS THE LAST TIME YOU TUNED IT UP?