I've got a PIX 515e running FOS 7.2(4), and there's currently 48 entries of one crypto map. Half of these crypto maps connect to Linksys BEFSX41 VPN end points, and most work really well. However, every once in a while, a random tunnel just simply drops (never the same one twice). My Syslog server shows, "Failure during phase 1 rekeying attempt due to collision," but I've checked the Advanced settings, and the renegotiate times are accurate.
On the PIX, my ISAKMP time setting is 86400. The crypto map time is 28800.
On the Linksys, Phase one is 86400, and phase two is 28800.
Both devices run DH group 2 and PFS with group 2.
Restarting the Linksys definitely does not work, but removing a line from the crypto map statement on my PIX and readding it gets the tunnel up again.