Since you are seeing blurred vision only episodically, you are likely to be having Asthenopia, or eye strain.
This is when the muscles of the eye become fatigued after long straining and fail to focus the image.
Some of the common muscle conditions which cause this are Decompensating exophoria, convergence insufficiency, etc.
Orthoptic exercises or eye exercises are usually sufficient to correct this problem.
You can do some eye exercises for this: http://www.coopereyecare.com/eye_strain.htm
However, considering your age, you are also likely to be having eye problems such as Presbyopia.
You must get yourself examined by an optometrist or better still, an opthalmologist. Sometimes other causes like optic neuritis, glaucoma, etc, can also cause such episodes of blurred vision. Neurological conditions may also cause this for which you need to see your doctor. Of course, you have mentioned that you are getting your eyes checked regularly, so perhaps you have been examined already. In that case, it will be sufficient to continue the exercises.
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