Mazda 6 V6 2006: having surging issuez

I'm a new owner of an used Mazda 6 it is a
v6 3.0 automatic 2006. My dad got it at a
dealer when they had an as is action for only
$1000.  The car is in good condition but has
a surging issue, and a engine code  that says
it is miss firing. I got the sparkplugs to
replace them but I am thinking it is more
than just sparkplugs. When it is surging
(never in park or neutral) but in gear either at
idle or when accelerating it surges/ bucks. If
you listen to it when it does this I swear you
hear a grind or cranking sound. I've checked
the fluids all is good but I think the
transmission is a little high( above the full
line by maybe an 1/8 or 1/4 of an inch.
Wondering if anybody could help