Posting a me too @mike Jolley with some (limited) value add ??
Have seen this issue several times, and also watched the various threads on this subject with interest, since admin-ajax and the more recent ?wc-ajax=get_refreshed_fragments end point.
What almost always resolves this issue are, in this order:
1) Host. Usually some bad host setup (puny memory allocation, other old or otherwise misconfigured setup). Change hosts. See if problem persists. In case of doubt, setup an AWS EC2 instance with only the stuff on it needed to run WP + WooCommerce. There’s a couple of AMIs with this on. 90% of the time, you’ll find there’s no issue.
2) Plugin conflict / bloat. Some plugin or theme file is bloated or a conflict exists. Try disabling everything, and using woocommerce with the default WP theme. If your issue persists, it is probably point 1 ??
Posting this as I’m a little tired of reading that this is a WooCommerce issue, when its really not.