Any Ideas?

Hey guys, I've got an interesting problem that doesn't seem to make much sense (to me, at least). I have a 6309-8210 with a 6309A movement that is running very slow. The "slowness" is not always at the same rate, sometimes losing 5 min/hr, sometimes 45 min/hr. To be honest, ithe watch may simply require a cleaning because I don't know its full history, but I wanted to throw the symptoms out there in case there was something that stood out as odd.

-- I'm pretty sure it has a good wind on it because I shook it for 5 minutes.

-- The weight rotates freely & easily and I can hear the mainspring being wound in both directions of weight rotation.

-- The balance wheel rocks back and forth quickly and at a regular rate. If I physically stop the balance, it immediately starts up when I release it.

-- When I manually set the time the hands glide smoothly and correctly in relation to each other.

-- Originally I thought that the second hand did not stop even when the hour & minute hands did, but I just saw it stop twice, in the exact same spot, several minutes apart.

-- The Day/Date wheels operate smoothly & properly and at the proper time in regards to the position of the hour & minute hands.

Thank you in advance for any help.