Transponder signal loss



I have an Echostar D2500IP receiver with Arcon sweety dish and am having trouble picking up signals from certain transponders on Astra 19 and Hotbird 13 (eg 12092). Sometimes I get no signal and sometimes a signal for a few minutes only before complete loss of signal. I get good signals from other transponders on the same satellites. Is it likely to be the LNB - it must be about 5 years old by now?