80k? Time to replace them again. I'd say there's a good chance they are causing the misfires.
A lot of times fouled plugs will cause a jerking when holding the throttle steady, or slowly accelerating, between 40-50 mph. When my truck needed plugs, it also had a bit of a clunk. At this point, I would definitely say that the plugs are prime suspects. If you put some nickel anti-seize on them the first time, there shouldn't be too much trouble getting them out.