One of the more common reasons for vacuum pump failure is not the pump itself, but the check valve. It is located where the vacuum line attaches to the pump. A bad check valve will cause intermittent problems like you describe. The good news is that you can replace the check valve while the pump is still installed on the engine. If I were to rebuild the pump, I would replace the check valve on principle, anyway. (It is not included in the valve kit.) The pump is very rebuildable, unless the housing is cracked, broken or otherwise distressed. You will not know for sure which O/H kit to buy until you remove the pump. Most likely you will need only the valve kit, unless the rocker arm assy is in obvious distress. The rebuild is not difficult, although getting the rocker spring compressed during assembly can be a challenge.