Well, since you got a new one, and the new one acts like the old one, then it must be correct. My next guess would be that you linkages are out of adjustment. Here is a link to the manual for a GT 2554 (same thing). On page 22 it gives you the procedure for adjustments if you want to try that route.
http://www.manualsdir.com/manuals/65...4.html?page=22