system crash and missing firewall log data: hacked?

For a while I thought my iMac Intel Core 2 Duo had been hacked so I did a clean install, reinstalled all software from scratch, and transferred my personal files one by one from an external hard drive. I also reinstalled my two Airport Express boxes and set up a new wireless network. All seemed well for a couple weeks. Then I experienced a total freeze-up and had to shut down using the power button. In checking the logs later, I noticed a 3-hour block from the Apple Firewall Log (appfirewall) is missing, up to right after I restarted the computer a minute after turning it off. I had been running Second Life for three hours before the crash. This is an application that usually hammers the Firewall from two different IP addresses throughout the time it is being used. There should be three hours worth of hammering in the log, so it looks to me as if someone hacked the computer, erased a 3-hour block of the firewall log to eliminate evidence of their entry, and then froze the computer. Other logs look all right. How can I tell if I have been hacked? Are there other possible causes for the freezing up and missing log data? I can paste whatever portions of whatever logs seem relevant to determine what's going on. Many thanks!
Bodine