Why are certain web sites not accessible during the day?

At certain times of the day web sites such as Facebook, Craigslist, azlyrics.com, and www.itslearning.com cannot be accessed by our campus.  It is always the same sites at the same time; however the time of day and the amount of time they are not accessible varies.  DNS always resolves the addresses.  I've tried switching to Google's DNS Servers, but that does not solve the problem.

Here is my traffic flow:   ISP (Comcast) <--> Juniper JSR2320 <--> Cisco ASA 5520 <--> Campus LAN

Initially my entire campus shared one NAT'd IP address (using PAT), but I
noticed other devices that had a static NAT were not affected by this
problem.  Through vast amounts of troubleshooting, I segmented my campus
into different groups and gave each group its own PAT'd IP address.  This
problem only affects one group which is currently PAT'd to 192.168.1.2 (not the real IP obviously)
When the problem was occurring, I changed the group with the problem to a
different PAT'd IP address.   I then put a laptop outside the ASA firewall
with the 192.168.1.2 ip address, and the laptop still had the problem.
To me, that test eliminated everything except the J-Series, my ISP, and
the affected sites.

When the problem is occurring trace routes to the sites fail at various points either in or beyond Comcast's network.
Comcast is telling me they are not blocking anything, and one of the affected sites has responded and said they are not blocking us.  I'm at a loss as to what could be causing this problem.
jeffgoodAsked:
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Paul MacDonaldDirector, Information SystemsCommented:
I expect you're using a proxy server for Internet access and it's being configured to block certain sites at certain times.  This may be by design, or due to a virus of some sort .  Does the problem exist if you change browsers?

Alternately, can you verify you can get to the affected PATed address from outside while the problem exists?  This may be a routing issue somewhere way upstream.
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jeffgoodAuthor Commented:
We do not have a proxy server.  The problem will affect every computer that is tied to the affected PAT'd address.   When the campus was sharing one IP, every computer on campus was affected.   When that happened, I gave my computer it's own static NAT address, and then I could access the sites.   We have multiple OS/browsers affected

I will attempt to route to the affected IP when the problem occurs.  What is odd though is that we have an entire class c subnet and we are using BGP as our routing protocol.  However, only one IP out of the subnet will be affected.   I would think a routing problem would affect the entire subnet.
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Paul MacDonaldDirector, Information SystemsCommented:
As would I, but who knows how the upstream providers have their routers configured?  It may be that one subnet is NOT part of the BGP definition, or that it's somehow misconfigured.
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jeffgoodAuthor Commented:
This problem just went away.  Guessing it was misconfigured upstream routers.
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