Thanks for the info Peter.

I'm not sure it helps yet with the live site, but it seems to get me on my way with the sandbox. If I don't include the close header, PayPal's sandbox test IPN process seems to get hung up (the web page will hang) and eventually tell me it couldn't connect. With the header it comes back immediately with a successful send and I do get everything back to my IPN listener.

Next up I had an issue that PayPal's sandbox itself was adding carriage return and newline to their response code for which the live site does not. Trimming whitespace fixed that and it seems to work.

Now I'm going to shut down the live site to external traffic and test this out there.

EDIT: Tested later the same day that it was OK and went live the next day. Has been working well ever since.