I went back and re-read your first post and from that it sounds like your timing is retarded and your idle speed is set too high. 900 is way too high. The fact that is shuts off in gear leads me to suspect retarded timing. Make sure you get a dwell meter or at least some feeler gauges for the points. Without the points set properly, your timing will never be right. With everything stock, you should have probably 17hg vacuum as a minimum and more like 19-21. That is of course with the tune right.