I have this problem but it's not that serious. Mostly just happens when I back up but I think it's do to my Ecam. It doesn't really die when I go into a spot, just on the reverse, I'd say 25% of the time.
A little unrelated but I had a similar problem in the 4-speed of my '71 Formula... The clutch was fine most of the time but if I pushed it all the way in, it would die out sometimes. I had some work some in the wiring and my shifter replaced, so I am assuming that it had something to do with the transmission itself. The problem just went away, the mechanic never mentioned how we fixed it. I ALSO had a really lopey cam that would still sometimes cause it to die.
So the question becomes... Do you have the stock cam still?
are you in gear coasting with the clutch in when this happens? is it like you have it revved a bit, then push the clutch in, let off the gas all the way, hit the brakes (all at once), and it dies?
what is your idle set at? does it do it with the a/c on? (im thinking the IAC is not catching the drop in revs and load). the puter should [IIRC] monitor that the car is rolling and drop the revs down slowly).
clean the IAC?
i dont know that i would mess with the timing, other than to time it just to know what you are running. you might not want to introduce another variable, unless your timing is insane.
i would continue to eliminate/try things to diagnose this (and articulate these things to us).
like, i have had cars that marginally idled as it was, and if turning and hitting the brakes, the thing would try to die out. this only happened at night - i chalked it up to electrical load ( i did not want to mess with the IAC, as the puter tries to lower the idle and gets funky. bad surging. BTW, not a stang).
the IAC is the cylinder deal on the front of the TB. it is what allows air into the motor when the butterfly is closed. it should kick the idle up when the a/c is on (why i asked). it can be checked with a dwell meter, but that is impractical. most just clean it.
you should disconnect the battery after adjusting it, to zero out the puter.
i might try messing with the idle if you feel like it (or you have done it before and it is not original). quick version: warm motor. adjust the idle with the screw. once happy, reset the TPS to <0.97 volts.
disconnect battery and drive.
for a vac leak: a stock-ish motor should have around 15-20". a vac gauge and driving around will let you know how it is doing.