TimeQuest sdc problem

Hi,

in my design I have a number of PLLs which are merged into one. The problem is that the particular instance which appears in the netlist following compilation moves around. Sometimes it is one and sometimes another. This causes difficulty in my sdc file because I have to set a clock group but can't reliably predict which one will appear in the netlist.

Is there a way to get at the correct instance without having to report clocks every time I compile?

Thanks and regards,

Dave.