Buy a Mityvac vacuum pump and pump vacuum at all your vacuum units, to check if there's an obvious problem. If all ok, check the system bit by bit, blocking off segments of it with golf tees at the connectors. Block off all door units, then bring them in one by one until it gets worse... also consider buying a manual transmission version, so at least your gearshift isn't affected by this inevitable problem - it started to drive me crackers after 18 months of constant small to major failures in various parts of the system.

One of worst ones is rear tailgate - it has to flex when going thru the body into the tail, so try blocking it off where it joins under the carpet on top of the rear wheel arch and see if it improves