Im using the settings as recommended by others(10 amp/deep cycle mode) on my trolling motor battery but the needle on the charger guage keeps jumping from the middle,to the fully charged mark,and the fully charged light keeps flashing on and off as it does this,    what could this indicate?

my experience with this indicates it is fully charged and you should remove the charger.  This is also about the time i bought an onboard charger with a float mode.  Never looked back.  It is just too convenient this way

How old and what kind of shape is battery in, has it been stiing a long time without being charged?  Might want to take it and have it checked.  My first guess is the battery is no good.   Try the charger on your car battery and see how it does.   It should gradually come down unitl it makes the full charge lamp come on and then stay there.  If it does the same thing on another good battery, take the charger back.