Change channel, lose picture. power off...

2 years old, picture still fantastic. When changing channels or going from Direct DVR listing, screen goes black and changed channel/program picture does not come on. Sound changes to correct program/channel. Have to turn off TV power and turn back on to make picture appear. Then it is perfect again, until another channel/recorded program is selected. Then same thing. Always comes up, picture always perfect. What's going on ?