engines surges at idle

I have a 1994 Isuzu Amigo. When I first start the car it runs perfect but when it reaches operating temperature it surges at idle. If the engines comes just above idle it does not. The surge is so even it is like a pulse, surging about every 2 seconds. I'm not sure of the engine size but there was only one inline 4 cylinder engine offered. This vehicle is a stick shift. Thanks, Sharring