Hey,
with 2.0.1 the NAT is not a problem anymore. If you still have the problem, maybe could be a "traffic shaping" queue. I mean, the CARP traffic can be dropped under heavy traffic, and this can bring to an inconsistent CARP status between the master and the slave box.
I am not sure, I figure out that could be a traffic shaping problem today… this is my post:
http://forum.pfsense.org/index.php/topic,45045.0.html
Ciao,
Michele