I'll say it again... and it requires almost NO effort.... Put a new plug in and bum a tank of fresh fuel. It the problem doesn't go away, then look for problems elsewhere. Others have given outstanding suggestions but until you've tried a new plug and new fuel, you shouldn't try trouble shoot further. I've "learned" this lesson about 30 times over the last 40 years.

The problems you describe sounds like fuel that has gone bad.

Let us know what you find.

Bill