hi joe
I have the same problems on my 1812 router and on my 871 router. I have IOS Version c870-advsecurityk9-mz.124-15.T10.bin runnning.
Which IOS do you have active ?
Since this IOS is installed I have the problem that VPN Tunnel is showing active(show crypto isakmp sa) but from time to time packets are encrypt, on the other site not decryp. Do you also running this behaviour ?
thanks