Another thought, your secondaries may not be closing fully. This could cause a higher idle speed and a lean A/F ratio at idle, also causing stumbling.

If you have a centrifical advance I would check the weights to see if they are sticking (keeping the timing advanced at idle)

In addition to what has been said,,,my 750 holley would go past the point of idle and start to open again(but in the wrong direction) and idle would be up300 or so. Didn't matter where the idle adj was set. I tend to think it's timing related since it's dying at 1000 rpm. What is your initial timing?