I had this issue with my '08. It would do it and then quit and then do it again. I checked all my vacuum lines and then noticed that I had a crack in the intake tube for the top vacuum quick connect. Pulled out the factory quick connect fitting from hose, and slipped it on what was left of the tube and clamped with a hose clamp. Worked fine. I have since then changed the intake manifold out.
'08 GT; JLT-3 CAI; GMS 62mm TB; FRPP Hot Rod Cams; Headers; Excel super coils; Ford Racing intake; BAMA SCT4 programmer; Axle back Exhaust; off road X pipes; 4.10 Gears; TCI street fighter shifter; 275/40/18 rear tires; 1.5" wheel spacers in rear; Glow Shift Gauges in triple pillar pod; underdrive pulleys, 1.5" front end lowered; shift light; racing harnesses on both sides; tower brace; Ford Racing valve covers; JLT oil separator; adjustable panhard bar; Ported/polished heads, many more mods