|You mean 'operating system not found'? Does it do that when first booting up or do you mean it's running OK and then later you look at it and see that message?|
Assuming the former, that means it can't find the system files on the drive it's trying to boot from. Since you eventually get it going it sounds like your hard drive is slowing down. On some computers there's a 'hard disk pre delay' in cmos/bios setup that allows more time for the drive to spin up and be recognized by the bios. You may want to check to see if yours has that option. If so, set it to about 6 seconds.
Probably a good idea to run a diagnostic on the drive, although sometimes when a drive acts that way a diagnostic will tell you it's OK. That's because a diagnostic can't test the drive's initial state when booting up; it can only test it when it's running OK. But a diagnostic should be available from the drive manufacturer.
If it's giving that message suddenly after running OK for awihile it sounds like the drive may be failing. Again, try the diagnostic.
The mantra of the aging hippie--"Power to the government, right on"