the "test" account authentications come from the CSMon service. It fires basic authentications at the RADIUS and TACACS services - I say basic as these are hidden users in the internal db.
Basically its designed to test that as well being alive, the ACS is capable of authentication.
If the ACS server is really busy (ie at capacity) you'll see these messages because of timeouts etc.
VPN users who are being re-authenticated for any reason may loose connection if the ACS isnt responding.
Sounds like you have an underlying problem that needs investigation.
