Marnier 115hp stalling and idle issue

I have an 115hp Marnier outboard (not sure of year approx in 1990's) that is having issue when idle and at trolling speeds. I just got it back from the marina after spend $800.00 to tell me it was fixed but still same issue/ They replaced the spark plugs, fuel filter, lower unit oil and also they said the spark boxes needed to be replaced. My question is if anybody can help could this be water issue or vent issue? I have noticed liquid in the vent line enough to fill a water bottle cap a few times but nothing major. I have used the starton enzyme treatment and it did seem to help. If I am half throttle or even full the boat runs great and very fast. If I am at 3/4 throttle and slow down to neutral the boat will stall. When I come into the dock I have to give it more throttle as it will stall. If I idle at the dock sometimes it will stall in 2 minutes or 5 minutes when idling. I have talked to so many boaters in my area and all have different ideas so I am looking for some further advice. Is it waster, vent issue or the idle need to be tweaked? Oh I did speak to the marina and they said when it was at the shop it ran great and didn't stall. Thank you for any help or tips you can provide. My gaol is not to take it back to the marina that didn't fix it the first time