Did changing the plugs have any effect at all on what you're noticing? How old were the plugs that you removed, and did they look to have normal wear and condition? How about oil in the plug wells? Oil in the wells is very common in these engines, due to valve cover gasket leak, but lots of oil over time can rot the plug wires.
Have you tried pulling and holding the throttle wire to get a constant high idle, to simulate your "cruise" situation? If you can duplicate the hesitation, you could pin the wire with a vise grip, and then see if moving wires around changes anything.
One other thing that happens on our Aveos is the throttle body gets gunked up. You can clean the TB and see if that makes any difference. Unfortunately, and as always with these things, there are LOTS of other things beyond what I mentioned that could be causing this hesitation.