Sharp HC-4000 contrast issue

Something strange is going on with the HC-4000 I got. When your using the unit, after say 10mins the contrast will either slowly begin to get brighter or darker. Its fixable just adjusting the contrast. But it keeps happening and sometimes when ya first turn on the unit the contrast is bright and others its almost all black the screen. But as I said ya just adjust the contrast controls (keys) back. But I was curious if anyone else with a greyscale Sharp has had this issue ?

-Walt B