I believe Jnaas2 is probably right in his diagnosis of your problem being a defective dip tube. The link below is an excellent information page that includes instructions for a vinegar test on the debris, and a serial # listing of all the affected makes and models.
Rusty Layton, ASHI Certified Home Inspector, Defective Dip Tube
Sadly to say all water heaters affected by this defect are over 10 years of age and are in the twilight of their expected service life. It may be wisest to opt for replacement if indeed yours is affected.