Weird start up problem

[CraftyFs]

I have a 1998 ranger Xlt I4 with a manual transmission. When put it in nuetral and just let the clutch run and it starts just like a normal engine, but after a few seconds (maybe 10) the engine seems to sputter down and chug at a slower rate causing the vehicle to shake. After a few seconds of this the engine seems to go back to the normal hum and slight vibration that is in any truck or car. This pattern seems to just repeat.

Does anyone have a suggestion to what the problem might be? thanks

 

[99RangerOH]

I'm guessing Idle Air Control Valve, have you looked at it, cleaned or replaced lately?