Very low idle question

My 1990 b2600i truck will start out ok, but then after about 10 seconds it will go from 650 rpms to about 200 and rough idle. any explanation to this possibly. thanks. this is without the clutch in or gas pressed. just sitting there.

650...raise that idle son! mine is exactly 1grand and it does fine. i think that when u go allllll the way down to 250 its like green horning your truck, your motor is simply stalling because of your low idle setting. -bf

it may not be as simple as raising the idle, check your fuel filter, it could be stopping up once the fuel pump pressureizes the lines since it is an injected truck, or the fuel pump could be going out, or you could have a vaccum leak. check out these things first, then if they are ok i'd adjust the idle, they are supposed to idle @ 850 rpms not 1000......