evinrude ‘98 special (gas milage, and stalling problems)?

i have a 1998 evininrude 90 horse motor ('98 special) and i can run it all day and turn it off for not even a few minutes and i will have trouble starting it again. after a while it does start but then its hard to keep running even if i let it warm up for a few minutes, when i put it into gear it slowly stalls. however, once i get it going and moving, it won't stall or skip or anything, also it gets very bad gas milage (about 1MPG) for being on a 24ft. pontoon boat.