The engine may be overheating.
The Fix: Check and repair cooling system.
The fuel pressure regulator may be operating at too low a pressure.
The Fix: Check fuel pressure with a fuel pressure gauge. Replace fuel pressure regulator. (Generally not a DIY job)
The ignition timing may be set wrong.
The Fix: Adjust ignition timing.
There may be some type of ignition problem.
The Fix: Check and replace distributor cap, rotor, ignition wires and spark plugs.
There may be a fault in the computerized engine control system.
The Fix: Check engine control systems with a scan tool. Test circuits and repair or replace components as required. (Generally not a DIY job)
There may be a vacuum leak.
The Fix: Check and replace vacuum lines as required.
You have a bad idle speed control unit.
The Fix: Replace idle speed control unit.
The alternator may not be working properly.
The Fix: Replace alternator.