have you tried to bleed the car while its up on ramps? that gets the rad cap hole higher than the top of the lim. when on the ground level the cap is a bout a inch lower than the top of the lim.
the way i bleed my car is, rad cap open, idle till the fans come on, if the rad level drops while waiting for the fans to come top it off as needed. then when the fans turn on i open the bleeder valve till steady stream comes out. ( 3 to 4 full turns) close it, top it off, and cap it. and im always good after that one time.
waiting for the fans to come on idling can take up to 15 minutes, but you know the t stat is open if the fans are on.
any chance during any of the times you changed the t stat, did you make sure the bleeder valve is not clogged? im thinking the bolt you turn will just keep on unscrewing till it comes out altogether.
