Hi John,
There are several possibilities that could be the problem with your tree. Take a look at this first site and see if any of the problems look like what is going on with your tree.
http://agebb.missouri.edu/pdc/trees/maple.htm
For this second site, click on the left for 'Shade Trees'. Then you can click on either 'Whole Tree' and/or 'Branches/Twigs'.
http://www.agnr.umd.edu/users/hgic/diagn/
If neither of these sites are helpful, I would suggest that you call your local extension service. They may have you bring them a sample.
http://www.ceinfo.unh.edu/cesites.htm
Good luck,
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.