Zoho CRM API issue - The underlying connection was closed: an unexpected error occurred on a receive

Zoho CRM API issue - The underlying connection was closed: an unexpected error occurred on a receive

We have a .NET 4.0 custom Windows desktop application which is making Zoho CRM API calls. We intermittently get this error when calling the Zoho CRM API:  the underlying connection was closed: an unexpected error occurred on a receive

It does not appear specific to any particular call and it seems very random.

A lot of Google searching seems to point to TLS version, but Zoho has not yet forced requirement of TLS 1.2. So we believe this is not the issue. We are also not exceeding our API call limits. Any ideas?