2004 Oldsmobile Alero

The car runs great but at a light or stop sign when the brake is on the car idles rough. In park it does not. Could this be a motor mount problem or something else?

it sounds like a vacuum leak inside the brake booster. . does your brake pedal feel like it funny, also do you have to push harder on pedal to make a quick stop? does you pedal slowly move to the floor if the brake pedal is held on for a few minutes, you can try this . DO NOT DRIVE CAR AND DO THIS TEST. put emergency brake on it does hold ok? take the large vavuum hose off the booster plug it with something do not put something that will make the hose expain and make it loose when it is put back on.have some one just place finger off it. start engine get the idle in hot mode push on brake pedal. if the idle is ok . change booster.