Sounds like a power supply problem.
Turn unit OFF and disconnect the input dish lead.
Measure the Voltage there at switch ON. either 18V or 13V.
If stable for some time, repeat at the Dish LNB.
That should determine whether a shorted cable/ faulty LNB, or if the fault is in the decoder itself.
Also try the Menu on the TV itself to make sure it is OK.
Kindest Regards, " The Druid ".