I30 over heating

I have a 2001 I30 that overheats after about an hour of highway driving. It runs with the temp gauge at halfway until it starts to heat up. I have replaced the thermostat twice and flushed the radiator, to no avail. If I turn on the heater full blast it will cool down to almost halfway on the temp gauge. It is not losing antifreeze. Ran a diagnostic but nothing could be found. Where do I go from here? Thanks in advance!

Air bleeding has not been done? Air bubbles join/form steamlocks, pockets, causing overheating and may ruin/warp heads.

Either run engine over 2k rpm for an hour, cabin heater hose bleeding/dripping (full heat) and/or follow your maint manual bleed procedure... check top left Favorite Links http://www.cardomain.com/ride/748507/8