Ben,
We've seen people with similar problems when migrating to and from 3G/wifi.
As far as I'm aware the fault is not on anyconnect side but underlying network, but just to be sure.
Can you make sure that debugging is enabled.
Anyconnect -> Statistics -> Diagnostics -> Debug Logs -> On
Now once some experiences the failure, just email the logs.
The best would be to open up a SR.
BTW, I assume that you've checked this with 2.4 .4019 ? It was release a few days ago.
Marcin