Which NIC of ACS is connected to the switch (top or bottom), recommended is bottom.
Have you made any recent changes?
What is the patch level of ACS? If there is no patch then please apply the latest patch for code 4.2 that is 17.
Apart from that Could you please turn the services logging to full under System Configuration -> Service Control.
When the problem happens again please collect a Package.Cab. You can get this from System Configuration -> Support but clicking "Run Support Now."
Rgds, Jatin
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~Jatin