Im using 2930 and also have VPN connection dropouts ...

I have swiched from pptp to ipsec on lan-to-lan connection and it seams to work fine, but still all vpn pptp passtru uses are disconected after couple of seconds / minutes ...

Any suggestons ?

Yes, having the same problem with a 2910 connecting to a 2820 using PPTP; tried IPSec but couldn't get that working at all. The latest firmware update killed the 2nd Subnet (public) which gave me no connection at all. The only way was to go in as a dial-in user with PPTP which connects fine, works for a few mins then stops but the connection is still present. I then have to drop the connection and reconect then it works fine for a few more mins before dropping out again...