Does it nearly empty the sump when it runs, or does it only pump a little? You don't want the pump sucking air, but how low can you go? Are you sure all of the gutters and any other sources of water are directed far away from the house?
Sounds like your water table is high...there may not be much you can do if you have all of the sources directed away from the house properly. The place where it is being pumped to, is it higher or lower than the house? You'd think 50' would be far enough, though.