Hi Roland,
One solution you might find useful is to timestamp the data in the IG side on arrival, instead of the host side.
Assuming you have a fixed transport delay, it can give you an accurate timestamp to work with.
The advantage with this approach is that both timestamps (entity calculation time and start of rendering frame) are taken on the same machine, which will reduce the deviation introduced by using different clocks.
It will require some changes, as running a async communication thread in the IG to capture CIGI messages on their arrival (and add a time stamp to them).