I've been having this problem for months and its driving my users nuts.
When a user connects using the VPN client, connection times out on the client side after only a few minutes.
If I user ssh and connect to the box and run anything that generates traffic, the connections stays up.
Pix 515E version 7.2(2)
VPN Client version 4.8.02.0010
timeout values are set to 1hr
Transport is IPSEC over UDP NAT/PAT
Peer response timeout is 480 seconds
Local linksys router router has
IPSec, PPTP, L2TP pass through enabled
Windows firewall has exception for VPN client