We have a client with two major sites which are linked by an IPSec VPN (100Mbps WAN at both ends) using two 3900 routers. Since they were both upgraded to firmware 1.4.3 we've seen occasions where there appear to be as many as four connections between the two routers rather than just the one connection. When there are multiple connections the link basically grinds to a halt. Both routers also host multiple IPSec connections from other satellite offices which are have 28xx routers and DSL links to the Internet but these don't suffer any problems.

Any ideas?

Weve had nothing but bother with 1.4.3

ended up rolling back to 1.4.1 and more recently upgraded to 1.4.4 which have been so much more stable.