Help with code p0307

I have a 99 Suburban 1500, 5.7 with 113,000 miles. I had the intake gasket replaced about a year ago. The vehicle then started misfiring when I pulled a trailer. I started getting a p0430 code and had a friend with a scanner read it, he said the misfire history is Cyl. 1-0, 2-298, 3-11, 4-1193, 5-341, 6-0, 7-917, 8-0..He reset the computer and I got a p0307 code for misfire on cyl. #7. I had the wires and plugs replaced when the gasket was done. I replaced the fuel filter about a year ago. When I drove it last it misfired when it was cold. I drove about 15 miles turned it off then when I drove it again a few minutes later it ran fine and the check engine light p0307 went away. My friend said he believes the CAT is okay due to the readings on the 02 sensors. Any help is appreciated.