have 99 4 cyl. FI. when trk. start,all is ok. When it starts to idle down. It almost cuts off, idles up,slows daown and shakes, for about 2-3 mins. then levels out. seems to run ok. But idles down very low and almost cuts off. This up and down has no rhyme or reason. Other than at statrup. Have replaced idler air control on back of throttle body. And fuel pressure regulator valve in tank. Any ideas before I start replacing parts. 196,780 mi.