91 Mercury Grand Marquis Bucking Issue

Ok here we go. Recently did valve cover gaskets obviously removed the upper plenum to get to the passengerside valve cover. Re-installed did Spark Plug Wires and Plugs. Started car up idles great! Going down the road at 20-25% open throttle vehicle bucks on accel. Re-Checked firing order and sparkplugs everything good there. I will add that I did clean the IAC valve with carb cleaner dont know if that would make a difference. Driveside valve cover was leaking oil dont know if MAF sensor may have sucked in some oil or what. Do you guys think an IAC will cause this problem if its not FUEL or SPARK? Checked vaccums and intake for leaks no leaks found. Any help would be great!

Check the EGR/DPFE system. If EGR valve is not opening, restricted, or stuck open, it will cause a bucking sensation at light throttle.