You have a vacuum leak somewhere in the miles of tubing running around the engine compartment and/or under the dash. The leak is probably in the tubing feeding the climate control system. There is likely also a vacuum reservoir associated with the climate system. Look there first. The leak could also be elsewhere and just reduce the vacuum available to the climate system.

BTW, the engine produces less vacuum at higher throttle settings - e.g. under acceleration or at higher speeds. Hence the intermittent nature of the problem.

The car will run and shift better once you fix this problem.