The C1-PLL current draw is higher than DRO type LNBFs. Maybe the GTMedia STB provides inadequate voltage and/or current. Add to this the quality of coax, length of coax run, moisture ingress, tight coax bends, coax damage, splices, connectors, corrosion...
What is the loaded voltage on both polarities when measured at the LNBF end of the coax (not at the back of the STB)?
Did you run a single temporary coax cable out to the dish and completely bypass ALL existing wiring and switch?
Have you bypassed the 22KHz switch?
Removed, checked, cleaned and reconnected each fitting?
Tried a different and/or higher current rated power supply on the STB?