Hi Andre,
Funny you should ask I saw very similar sessions (for which we're waiting for logs) on ASA 8.2.3 and anyconnect 2.5.
Typically you need to send 4 DPD packets to assume session dead... which should make sure 2 minutes ... not 5.
Would you be willing to run debug on the ASA to check the the DPDs are kicking in at all and at what interval?
Marcin