Not likely a DoS of the entry guards. Unless strict entry guards are used, the hidden service will switch to a new entry guard temporarily while one of its previously selected ones is down. The attacker would need to DoS a lot of entry guards simultaneously to prevent access to a hidden service. They would probably be able to brute force deanonymize it with mass entry guard DoS + timing attack before they could block access to it in this way. It looks very much like itnroduction node DoS. The behavior that you describe sounds like what I was seeing on the forums when they suffered down time, and is indicative of a crashed Apache. However, I am personally incapable of establishing a circuit to the marketplace.