1991 Mustang gt idle surging

My 91 has just recently started the whole "I'm a typical Ford and I don't want to behave when cold staring". This just started very recently, it surges constantly between 2500 and near stall out (approx) when I start her up first time for the day. Now it'll do this for about twenty minutes or until it gets up to full running temp then it's only happening intermittently for the next few mintues. Sometimes going as far as complete stall out while slowly accelerating up to traffic speed. Forget about quick launches or even passing gear accelerations for at least the first 5-6 miles.
Last night I put a timing gun on her and after pulling the vacuum advance pill I realized that without it, there was zero surges of any kind. I'm a bit baffled and could use some advice. Thanks in advance. Have a great weekend everybody.