Good news. After 2 days thinking it was my filter and on the phone with the filter co, then thinking it was my computer, and trying to restore system backups, and then finding the same problem on my other computer on the network, the only thing it could have been was either the router or something at the connection level. As per Tomim's advice, I called the ISP and they helped me reset the proxy to manual and put in a specific number and port. And B"H after that, ConstantContact seems to be working smoother and faster than ever before! (Another example of a Bracha in disguise). Sheesh. What a headache. Hours wasted on the phone with tech support. Nu, Gam Zu Letovah.
Alls well that ends well!
Good Shabbos GYE!