IIS4.0 and Proxy server stops responding

Has anyone ever seen a problem like this. For no reason and with no event these services just stop. can happen 5 to 20 times a day. All patches have been applied. Worked fine for years till about 3 weeks ago. Scaned for viruses, and none show up. Contacted norton and they don’t know of a virusthat can do this. Nic’s are in working order.