Seems impossible and very strange.
Look at
Change IP on SCO - it may help.
Make SURE you are saving the host file when done editing. Check that your entry is in there after finishing.
Do a ls -l on /etc/hosts - insure it's not a link to another file.
Do a checksum (command may be sum, checksum, or something else?) on the file after you edit it and are SURE the hostname and IP are in there. Save the info checksum gives you and compare it to the file after reboot.
You can put the hostname with a different IP AND the real IP with a fake hostname to see what it maybe looking at when it gets removed - this may help when trying to find what is deleting it.
Check your startup scripts - anything new? Did you recently change the IP or hostname (check the link for all the changes needed)?
Post back with anything further.