utl_http.request intermittent failure

On our 12.1 database we have setup a Wallet with directory so that eventually we can make this kind of call:

utl_http.request('https://some.server.com/some_test/services/ImportHub?wsdl') FROM dual ;

This works.

Now, upon using this call, it actually has a failure rate of about 10% to 20%

it then fails with either of these two sets of errors:

ORA-29273: HTTP request failed

ORA-29268: HTTP client error 407 - Proxy Authentication Required

ORA-06512: at line 104

ORA-06512: at line 119

ORA-29273: HTTP request failed

ORA-29259: end-of-input reached

ORA-06512: at line 104

ORA-06512: at line 119

I must stress again, that the setup does work. In most cases the call does work, and it does like intended. So, we can't be having a fatal setup error.