How old is your clematis? Usually it happens with young plants. Are you sure it is the wilt? Sometimes some creatures can damage the stems and they wilt. In my case slug do that. They damage the surface of the stem. When the shoots are young everything looks good but when it is time for bigger work - they collapse. If you really want to replace H.F.Young try something from viticella group (they are wilt free): Blue Andel, Emilia Platter, Perle d'Azure, Prince Charles, Justa. There are so many new varieties with viticella genes which are not susceptible to wilt and have pretty large flowers. I love Vancouver Sea Breeze. Actually most clematises from group 3 have viticella blood and supposed to be wilt resistant.
