Could be fuel pressure. Do you have an AFR gauge? You would see it lean out if it was the fuel pressure or the maf sensor or potentially a clogged fuel filter (super easy fix, you should do this anyways).
Could also be missing due to loosing a spark. You can test your wires, test the coil pack, replace the plugs, check for cracked insulators, carbon buildup, etc. How old are the plugs? . Often the most noticeable spot for a weak spark is at high vacuum for some reason. Theoretically it should have an easier time marking a stronger spark under vacuum.
The climbing vac gauge at a stop light is indicative of an increased load on the engine or a decrease in power.
Check your power steering fluid levels, alternator charging voltage, try turning off high power accessories like amps, make sure your fans work properly...