Originally Posted by Hit Man X

Last few cars I had that ran like ass at full throttle (shaking/vibrating/bucking) had ignition issues. Cracked/corroded caps, worn rotors, wrong plugs, excess variance in the wireset, burned wire ends, etc. I would think that if the car is missing at idle the vac will not be steady as the cylinder will not be cycling properly.

Vac leaks throw off the AF mix as the air is unmetered. My experience with older cars is HVAC problems and poor fuel economy. If the leaks are large enough difficult starting and hesitation at low RPM. Newer stuff throws a CEL almost always. Inspect all the rubber connectors by hand even if they look okay. I had nearly 1" loss due to a three way rubber 'Y' connector at the firewall leaking... it basically fell off when I touched it!

Just have to remember, low mile/rarely driven cars tend to demand a stricter maintenance schedule and that city driving (stop/go) is harder on parts as they wear even at low speeds (think hours versus miles traveled).