1980 Mercury 500 50hp spluttering & cutting out on idle

[Green Devil]

I'm new to boating & have a 1980 Merc 500 50hp on a 15 ft speedboat. It will start, throttle up & will get the boat on the plane no problem. when I go back on idle the engine jumps and revs up & down for a short while and then cuts out. It wont start again until i get it home which may be 2 hours or more. Need help on this one as I live in Ireland & not many good Mercury mechanics about and very expensive if I have to bring it to the shop.