Zen Cart is correctly using the required endpoint:
Live API Signature Name-Value Pair https://api-3t.paypal.com/nvp
PayPal has been having problems with some of their "3t" servers since their recent maintenance updates in the last couple weeks. You might get more attention if you keep complaining to them when the problem occurs.

Additionally, if the "cannot resolve host" message is constant, you may have a problem on your hosting company's webserver with respect to DNS lookups, and should be reporting the matter to them.