ISA flapping,

We regularly receive this message in the ISA server alerts tab:

The Web Proxy filter detected that the upstream proxy server '' is not available. 
During which period the internet is not working.

Then after 2 or three minutes we receive:
The upstream proxy server '' is now available. If you changed the configuration of the primary route to the upstream ISA Server, you might want to change it back.
and the internet works again.

when it starts happening it happens very frequently (the error is logged every 2 to 3 minutes) then it stops for a while where the error is not logged for hours or days, then it happens again.

What could be the reasons for that? the ISP says their server is up and running 24/7, what other reason could cause that.

Thanks for any help,
Who is Participating?
The best thing to do is look at the monitoring section of the console in ISA and check the logs before and during one of these outtages. You should be able to at least remove a few possibles.
Also see if anything is happening with the network card in the events logs during this time or isa events in app logs.
Tell us how you go or what info you find.
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