P0300 Multiple misfire

Hi everyone. I have 1996 3.5 engine. It has 70,000 miles. Recently I changed Intake Mainifold Gasket due to rough idle. While changing gasket replaced plugs and wires. Engine ran great for 10 - 20 miles then rough idle with engine light blinking. Code is 43 ( P0300 multiple misfire obd II). I changed plugs and wires again and no changes. What can cause this P0300. I'm going nuts here!!!
Thanks