Sounds like one or more problems going on. You do not mention the setup you are running i.e. open / closed primary, 4 speed etc.
The most common problems are improper alignment, improper tension and worn pulleys. A worn pulley can have sharp edges on the back side of the teeth that will cut a belt quickly. A belt that is too tight will snap under load and wear quickly. An out of alignment or a set of pulleys not tracking true to each other either by offset or not tracking correctly will wear the edge of a belt off quickly. An out of offset may only shave an edge off. Out alignment where one pulley is cocked off to an angle will tear a belt up fast.
Rather than repeat some pretty well written tips, click on the links below for some install tips. I know you are not running BDL but the install is the same.
http://www.beltdrives.com/guides-1.htm http://www.beltdrives.com/guides-2.htm
