Spurtering imo not vacuum spring related. sounds more like an advance issue
Stalling issue. soudns like you need the anti stall solinoid/dashpot.
you can bump up your idle about 300 rpm just to see if it stays running
1970 Mach 1 San Jose built Dec 23 1969. Marti says 1 of 7. Purchased in 1987. Original family owner of the powertrain 351C 2v FMX.
1993 GT 11,000 miles, Built 2-12-93 Auto, 3:27 Axle, cloth, sunroof. Untouched except for rubber and battery, Purchased new 8-3-93. still has the factory windshield fluid and new car smell.