Increasing episodes of double vision

I am a very healthy 63 year old female.  Over the last 3-4 years I have had increasing random episodes of double vision.  Originally it was once every few months and would last 20 minutes.  They occurred only when I was exercizing - usually walking quickly and never when driving or riding in a car.  These have slowly been increasing in duration and frequency.
During the last year they have begun happening several times a day and the duration is much longer- hours even.  It ranges from slightly overlapping images, to clearly double (2 birds in the sky - when there is in reality one).  There are no days when they do not occur now.  They generally come, then clear away slowly until the next time.  
During the last month or so they have begun to happen when I am still- reading or working on the computer, as well as exercizing or in the car and persist for longer periods.
I do have glasses with a prism and that does help clear it up.  I usually only put them on when I am having a problem, since when I take them off the problem is considerably worse for 30 min or so.
My concern is that this is a precursor of something more serious.  
My primary physician has sent me to specialists- I have seen 3 eye doctors in one practice (one for cataract surgery) and discussed this with all of them ( no idea of why this is happening),  one neuro-opthamologist, and one neurologist.  I have been tested for Graves disease, thyroid disease, myasthenia gravis (3 tests- one test was botched by lab), generally cbc was done, had 2 cat scans, and 2 MRIs over the alst couple of years. There have been no problems found- except a cataract in one eye was removed.
Should I let this go and live with it ( I can the glasses w/ prism do help) or is there a possibility of this being a sympton of a more serious health issue that should be further pursued?

[JodieJ]

I'm not an eye care professional, but it sounds like you could have a mild phoria which is decompensating.  This is not a serious health problem.  Have you seen a strabismus specialist (sometimes called a pediatric ophthalmologist, but they usually see adults, too)?  If this were your problem, it can be corrected by prisms or a painless outpatient procedure.

[John C Hagan III, MD, FACS, FAAO]

You have had a very thorough work up and should be thankful that your problem was not caused by a tumor, stroke, aneurysm, etc.

I agree with JodieJ that the next thing is to see a strabismus Eye MD to dicuss treatment. Ask your ophthalmologists for a referral.

JCH MD

[suew2424]

Thank you- my only concern was that there was a more serious issue underlying the problem. Although I was well checked, no one was able to tell me why this is happening.  

[JodieJ]

It's probably due to a slight imbalance of your eye muscles.  I also developed this problem as a middle-aged adult.  At least in my case, the outpatient surgical procedure worked great and was completely painless.  Prism glasses also worked pretty well, but the lenses were so heavy that they were constantly sliding down my nose (despite many attempts to adjust the frames).