Changed thermostat and fluids, still overheating

I suddenly had my car overheat. I checked fluids,
oil, tranny, and radiator fluid. Was a quart low on oil
(my bad) and the radiator only took about 20
ounces of coolant. I checked for leaks; head
gasket, radiator and hoses: they were all good. So I
changed out the thermostat... It's still overheating.
I thought temp sensor, but I'm not getting
diagnostic code or even a check engine light for it.
It heats up when I'm on the highway so I ease
down the RPM and it's settles to normal running
temp on dash. I tried turning on the heater vents to
help cool it, but that made the temp spike again. I
can sit at idle and it's at normal temp not matter
how long it runs, but shortly after I get moving, it
will begin to rise. If I leave A/C  going it will get hot
fast, if I shut off A/C it will slowly settle. I can also
drive through town (w/ all the stops and go's) and
not have a problem with the running temp. It
seems to be when I run in 4th gear that it wants to
overheat. What could be the problem? It's a 2004
Hyundai Tiburon GT V6.