strange network problem updating online accounts

[Jack]

For the past few days, every time I try to update an online account  
(libofx) I get a popup "Unknown host xx.yy.com:Host not found" but I  
can put that exact host name into Firefox and get a good result.  I  
also get no error using that name in a ping command.  Traceroute often  
times out before reaching the host, but it does get a good IP address  
to use.

This happens for all of my online accounts, including banks, credit  
cards, and investment accounts.

I'm at a loss as to where the problem actually occurs, since DNS is  
obviously able to look up the address, and the address can be reached.

Any other thoughts about how I can further troubleshoot this?

Jack