It is possible that your hold down is bottoming out on the block and not actually holding the dist tight like you think it is. Your oil pump is on the other end of that gear, do you have the pump drive shaft stacked up where you can not get the dist to the block? However all you need to do to check the hold down is grab the dist after you set your timing and see if you can move it by hand. Otherwise you may have an accuracy issue on how you are setting it in the first place due to a vacuum leak. Check for vacuum leaks, that will influence your vacuum advance that may also be hanging up some how and changing the initial setting? You may even have a power valve activating early in gear or blown and leaking that will kill idle quality big time.
Bruce