I would look at the diaphram before looking at the solonoid. It could be material not letting the diaphram re-seat or a worn diaphram. For twelve years, I would remove the entire top of the valve and replace with a new one of the same design (if you can get it).
If you can't find someone that has a Flo Pro, you might want to consider replacing the entire Valve instead of just the diaphram. Toro discontinued production of the Flo Pro valve several years ago. Most of the problems with the Flow Pro were that they wouldn't come on.