hot water tank emptying?

hi,
i have a hot water tank which empties itself when i turn the temperture up to 50 degrees from 45degrees.this did not always be the case.i have changed the tank stat and the pressure vessel.any ideas what could be causing it would be greatly appreciated.tank stat used to be set at 70 degrees up until a couple of years ago when we have steadily been decreasing it to stop the blow off.flow regulator set at 0.2bar and mains pressure set at 1.5psi.