one pipe water hammer

Al_14

One pipe steam system. New radiator vents. Manual water feed. During cold weather (zero and below) water seems to be accumulating in a feed going to one particular Rad. Water gurgles in that leg and radiator, also occasional water hammer. The Rad has heated but on occasion gone cold. the rest of the Rads work fine. The water tube at boiler starts off at the correct level (3/4 up the glass) but as the night wears on it slowly goes down to a point about 1/4 of the way up from the bottom. Then boiler short cycles because I guess lack of water. If I shut down the system to give it a short rest, (20 mins or so)the water trickles back to the correct level again. I have been reading about main line vents but I have not located any. Where would I most likely find the main vent? In one of the legs to the radiators or in all legs? How many of these main vents should be on the system? Could the main vent be the problem or clogged line to the Rad in question. I live in Hudson NH and the oil co guys that I have called are not interested in finding the problem. The old hit and run routine. Does anyone know of a good steam man that doesn't mind figureing this out. I would prefer someone close by.