You likely have a slow vacuum leak somewhere. The most common cause is the vacuum reservoir mounted under the passenger side headlight and the rubber line leading to it. Since you mentioned in your other post that this vehicle is new to you, I would also inspect all the vacuum lines in and around the engine to ensure that they're not cracked and/or damaged.
His: '00 WJ 4.7 Limited- Zone 4" SA lift, JKS BPE's
Hers: '03 WJ 4.7 Laredo- IRO 3", 245/75/16 Duratracs, Bilstein Shocks, JKS TB w/KOR Bushings, Rusty's A-Arm Spacer, CORE4x4 Adjustable Front Control Arms, Dynomax Exhaust, RB1 Nav -Coming Soon- Overland Rock Rails