When that happened to me, it was my horn ring going bad.  It had worn thin
and pieces of it were coming off and shorting it out.  Finally one day a big
chunk came off and wedged itself in there tight.  Had to get out and
disconnect the horn.

I think your first step is to pull the steering wheel and give a look under
it.  You can get new horn rings from various list vendors.  Oh, and search
the archives for "horn ring" and you'll get lots of info.

Bill Marshall
85 GL Tiico "Pandora's Box"
Aurora, IL