you cables and LNBF. may be issue on cables and LNBF. if you have multiple LNBF check individual LNBF. if every one giving same problem. then check cable from diseqc to your receiver. may be lightning effect.
it is better if you can check with another DM 500. then go to DM 500 check it with some were else from your setup.
if same problem there. Try with factory reset and reconfigure. if cannot re-flash box with PLI Image and check.