This also occurs for small payloads (e.g. list operations). Google's support team has acknowledged that the problem was on their end. They sent us this message: "I wanted to let you know we have some more information regarding the root cause of the issue you faced. Further investigation with our engineering team confirmed that the issue was caused by a provisioning error in the internal Cloud Storage infrastructure that led to low performance and “Service Unavailable” errors when handling uploads to the US region."

Unfortunately the problem is still occurring after I got this message (although less frequently).