I had the same problem for a few months with my 96 Civic. I went to the dealership and they told me it was some kind of vaccum leak. They said just to check all the tubes to see if anything is loose or leaking. However I found my problem to be a crack in the brake vaccum thingy and luckily fixed it by just patching it. I don't know if that helps, but just lettin ya know what I was told for the similar problem. Good luck man