Aside from the legality of shooting across a state road
I would think you should be able to get a little closer to them for an easiershot.
That being saidand getting back to your original question "is the rising heat from the road causing you to miss" I can pretty confidently say no. It's not going to be enough to cause a complete miss on a hog.
It's either operator error or your not sighted in properly for the yardage you are shooting. You state that you can kill the pigs out at 600-650 "easy" so I would assume you aren't holding Kentucky windage on them and your .308 is sighted in at that distance. If that's the case then you will be way....WAY off at 275-375yds.
Something ain't right and it's not the heat off theroad.
P.S. Stop shooting across state roads.