[ZOERA-SC7XX]

Despite what most everyone here thinks, I think you should connect your vacuum advance. I chased the same problem for a year before I connected it and instantly the car behaved well in the low rpm range. My initial timing is at 15 deg advance, add to that the 10 or so with the vacuum advance, (but not constantly). The mechanical advance will add to that up to 21 deg, so you may have to install the black button which will limit the total. Either way, there's lots of combinations to play with. The vacuum advance allowed my car to become quite driveable in city/traffic conditions. Detroit engineers were pretty good, don't you agree. It may not be the answer for the guys who track their car, but may be a good idea for general cruising.
I hope this helps. Good luck with that beautiful GT.