you do a lot of driving on it so it MIGHT not be the issue but those 3.1s can trap air like you wouldnt believe. what i usually do is this. warm the car up at an idle until it reaches operating temp then hold the throtttle at about 1200-1300 rpm. when it starts to get hot rev it up to about 2200-2400 and let off till it comes back down to 1500ish and rev it back up to 2200-2400 and repeat several times. you should be surging the engine like youre getting ready for a badass takeoff, in park
. thats how it should sound anyway. what youre doing is working the cooling system to try and purge the air since the high spots are hard to work the air out of. after you do this warm it up as high as it will go without overheating and shut it down and walk away. let it cool down for 30-45 mins and check the overflow. fill it back up tot he hot line and do it again. hope this helps. i know when i do anythign to the cooling system on those bastids 9/10 i have to do this to get the air out even after i opened the bleeders and whatnot.