Please see my past post on this topic, on which you also commented:
http://forum.pfsense.org/index.php/topic,48966.0.html
I have checked a number of times with my ISP on this matter and they can supply me with no information/clue as to what is going on, but on a few occasions I know I was not the only customer who went down. What I don't understand is why I have to do a complete reboot of pfSense in order to reestablish the PPPoE connection. I do use a PCI modem card (Viking) in bridge mode in my pfSense box for my DSL connection, so it is possible that the card is crashing. The interesting thing is that I do not loose DSL sync when my PPPoE connection goes down and the basic ATM protocol on top of PPPoE stays connected (I know that from my ISP). The only way I can reestablish my PPPoE connection is to reboot pfSense, then I always end up with a new (dynamic) IP address from my ISP. The routers I owned prior to building a pfSense box also all did the same thing, however, so the problem may not be with my modem card. The disconnects are seemingly random, but seem to happen during web surfing (not inactivity).
When my MTU was set at 1500 I could force a disconnect simply by doing an online speed test (apparently during the upload portion of the test). Changing the MTU down to 1492 and the MSS (box setting) down to 1452, however, allowed me to do online speed tests without crashing my connection. That is why I am wondering if lowering MTU/MSS further might solve my "random" disconnects.
The only other thing I can suppose is perhaps my ISP runs a program to periodically and automatically disconnect users like me who leave their modem/routers on all the time in order to free up a larger pool of available IP (ver. 4) addresses, but that supposition is merely speculation on my part.