NetFlow Analyzer stops collectiong data

Hi,

we have NetFlow Anlayzer in the Free Version (only 2 interfaces) running for over 1 year. Without any changes we ran into problems, that Analyzer stops collecting data and stops drawing graphes. A restart of the service, and sometimes a restart of mysql was needed, but only a few hours later, it stopped again.

What I've done:

-> Update to the latest version (8600)
-> Update Buffers of mysql in startDB.sh as suggested (--key_buffer_size=720000000 --innodb_buffer_pool_size=848000000), 2 GB RAM
-> Delete all old Data with dropDataTables.sh as we need only the last month for billing
-> Running myisamcheck over the netflow-database
-> setting the clocks of Server and Router syncron (both now with ntp)

...but the Problem still exists.
Who can help? I have no idea what to do to get Analyzer working again as stable as before.

Many thanks in advance!