WHEN I FIRST START MY 99 MUSTANG GT, THE IDLE JUMPS FROM 1500 RPMS TO 100RPMS AND EVEN STAHLS SOMETIMES. WHAT DO YOU THINK THE PROBLEM IS? IT IS ALSO MISSING ON HILLS AND DOES NOT HAVE THE THROTTLE RESPONSE NOR POWER IT ONCE HAD.
The most common reason for the idle drop is the IAC (idle air control) It is stuck/sticking and will need to be replaced. As for the lack of power. A simple tune up up and fuel/air filter replacement will help here. If it is a miss, the check engine light will come on and the codes will need to be scanned to pin point the misfiring cylinder.