Well, I've learned a few things on this journey:
Endian looks nice. However it's buggy, and unsupported.
Optimum Online sucks.
Pfsense has a lot of cool features I wasn't even aware of.
I missed pfsense.
I left Endian working over the weekend, and the connection was perfect over the weekend. So, I put in Pfsense to see if I could play with settings and get it working, but it worked fine, and has been fine for the past 24 hours. I was really hoping the problem would still be hanging around, so I'd have a chance to change settings and figure out what was going on. If anybody ever has an idea why this happened, I'd be grateful to know for next time.