I've seen so many complaints like this, I just assume that Coinbase is unreliable and shouldn't be used if you need quick response times.

However, in that case, don't touch fiat either..

edit: https://steemit.com/coinbase/@evanrvoss/coinbase-why-do-you-always-give-an-error-message-when-the-price-of-bitcoin-and-ethereum-drops