Mitsubishi Repair: montero sport, throttle position sensor, montero sport

Question
i have a 2000 montero sport when at an idle and the heat is on it will idle down and up back to normal then back down and up and back down till it dies the u can start it back up and it.

Answer
Jeremy,
This may be caused by a worn alternator or a malfunctioning idle air controller (sometimes called an idle speed controller).  When an alternator start begins to show signs of failure, putting extra electrical loads on the system (like A/C heater blower, etc.) can take power away from the ignition system and cause idling problems.

Evidence of a failing ISC is whats known as 'idle surge'.  This matches your description of idle bouncing and searching until it eventually levels off.  Other components of the throttle body which may contribute to this problem can be a bad throttle position sensor or a loose base idle set screw.  Have your mechanic check those areas and hopefully they'll be able to find and resolve the problem.
Good luck!