Even though it's only been 4 years, I would check your vacuum lines and check valves.
Good advice, I had the same intermittent problem some time back. Turned out to be two check valves were no longer "checking." I replaced them along with new vacuum lines, problem solved.
I even had to return two check valves for different ones, even though new they didn't function. Guy at the counter just shrugged, said go pick up two more so I did. When I returned to the counter he had already put the bad ones back in their box and was going to hang them back up for the next sucker. What he didn't know is I marked them and the boxes, then called the regional office about his actions, told them how they had been marked. Two days later they called me back, thanking me....the guy I found out had been let go.
