To me it almost seems that Coinbase does this intentionally. Such a huge company should be able to keep their website running even during activity surges, or at least prepare for these things in the future. But Coinbase crashes always (with 100% certainty) when any price dip occurs. It's like they're intentionally letting Coinbase crash so that people can't buy at low prices (which would be a loss for the exchange).
But this is just a conspiracy theory, they probably just have a fucked up infrastructure.