Thanks for the quick answer.
When I do a show span, there aren't any recent topology changes.
The network still works during a broadcast storm. During a storm the router sends many arp requests(500 to 700 broadcasts per second). Every request during the storm is for the same ip-adres. This adres changes with every storm.
My experience with loops teaches me that they almost bring the whole network down.
Spanning tree is configured in normal mode on every interface on the router. This should prevent loops.
The router is also de root switch for STP.
The arp table isn't flushed during a broadcast storm. When I do a "show arp" the current entries are still there before, during and after the storm.
I will try a firmware upgrade.
Thanks for your advice.