396 engine problem

[rustygmcc20]

so i have a 396 and this is the second time its started doing this but last night when i was pulling into the driveway and stopped the truck the exhaust sounded a little weird kinda like if it was misfiring and sounded like a 6 cyl and the exhaust smelled like gas but i didnt feel like messing with it since it was late and this morning when i go to crank it up it sounds perfectly fine, last time it started doing this i messed with the idle and turned it up and turned it back down and it was fine after that, but that was probably two months ago, im pretty sure its a miss fire but why would it happen every now and then and do any of yall know what the problem could be, sorry the post was so long, but thanks in advance for any advice you may have