Our ASA 5510 has sw ver 8.0(2) and DM ver 6.0(2). Through ASDM on the menu Configuration>Site-to-site VPN>Advanced>IKE parameters under NAT transparency we checked "Enable IPSec over NAT-T" and wrote config to startup and also we did it manually. After power failure and start ASA 5510 lost this part from config file. So we have to put command "crypto isakmp nat-traversal" manually. Because through ASDM it hasn't effect. It's happened two times.