We have seen this on several occasions.
At one location, we seen in the agent log that the line was bouncing randomly. Most or all machines at a location would drop off for 10-30 minutes and then come back online. The ISP was notified and ATT fixed the problem.
On several other occasions, agents would drop offline at various clients, various location, various times, and some would check back in while others would not. Finally discovered that now and then, when the VSA did its updates, some updates would disconnect and/or block agent connections. No pattern seen, which made it harder to drill down on. But, finally discovered that the VSA updates were causing the problems. Disabled automatic updates and have not had the problem since. We now manually apply the VSA updates and take more control of it to prevent the problem from happening again. Its so random, you spend more time trying to figure out what is going on than being productive.