I used the almost same config as
IOS router 12.0(7)T on 2621 (8MB flash)
PIX 515 6.2
the tunnel works fine during lifetime of 3600 seconds, then just after renegociation, it drops the connection.
the only way is to clear crypto sa on PIX and / or the router to re initialize the tunnel.
I tried PFS Y/N, I tried keepalive on both ends, always the same issue.