Something

item := nil;
while true do begin
   item := nil;
   with ResultList.LockList do begin
     try
       for i := 0 to Count-1 do begin
          pok := Items[i];
          if pok^.checking = false then begin
            item := pok;
            break
          end
       end;
     finally
       ResultList.UnlockList;
     end
   end;

   if item <> nil then CheckAvailability;
end;