There seems to be a problem in VFP's parser. cPeriod and cYear are both properties of thisform.oAD
Even the debugger shows valid values for all components of the failed line of code.

with thisform.oAD
locate for FinYear = .cFinYear and Period = .cPeriod && fails "variable cPeriod does not exist"
locate for FinYear = .cFinYear and Period = thisform.oad.cPeriod && works
locate for FinYear = .cFinYear && works
endwith

Any ideas?
I can work around it ok, but have I goofed somewhere?