Welcome to the Excalibur club. If your string is "jumping off the bow" it is most likely a brace height adjustment (it should be between the two lines on the rail)or that the end loops are to big. What string is on the crossbow? are the loops served or are they just twisted bow string?

You have purchased an excellent crossbow and the company really stands behind their product. They also have a terrific forum at: http://www.excaliburcrossbow.com/phpBB2/index.phpwhere you will also be able to get a lot of help.

Bob