Like hcgalvin said, if a tune-up has not been done recently, like spark plugs, plug wires, fuel filter, air filter and cleaning of the carbon deposits in the throttle area, this could be the cause of your problem.
If the above is ok, I would make sure I have new fuel filter. A bad fuel pump and/or fuel pressure regulator could cause intermit hesitation too.
Again, like he hcgalvin said, verify that the catalytic converter is not plugged.
If that does not work then try replacing the O2 sensor. A weak signal from the O2 sensor could cause your problem.
It could be a lot of things, so pulling the codes usually makes this task cheaper! I nearly replaced everything on my Toyota when it was hesitating until it finally went away. It was a combination of a lot of things (I figured). It was not throwing codes, but like you said, it would hesitate just enough that I WOULD FEEL IT, but maybe the passenger didn't.