Overheating

If the leak is on drivers side middle it is either the coolant crossover tube gasket (There are 3 of them) or the coolant hose that is attached to bottom of throttle body.  The reason that temp is acting as it is there is air in your system.  Maybe from a leaking gasket or hose, or someone changing coolant without properly bleeding out air from system.   Facing the front of your car left side top of radiator is a plastic bleeder valve on the 3.5 .  Read up because there is a special bleeding procedure for this engine.  You can find it on carguys forum.  Unlike most autos your thermostat it connected to the motor end of the lower Radiator hose instead of the top.  The thermostat and housing are one piece made together.  Expect to pay between $30 and $45 for it.  If leak is on passenger side middle check to see if waterpump is leaking.   Otherwise look at hoses and radiator for leaks if in front.....I hope that helps.  I have been all through an Intrigue 3.5 cooling system.  Recommend also a professional cooling system flush to get rid of the Dexcool (red antifreeze) and replace with the universal antifreeze ,but not the green antifreeze (it will eat away at aluminum) because you have and alum motor, and heads.   The reason to get rid of Dexcool is it builds up scalebuild up in engine and radiator that will plug up your cooling system.  After air has gotten to it the Dexcool starts breaking down rapidly.  There is much on the olds forums about this.