Doesn't the on/off button on the Center Cover Modula make contact to the Switch Key Board located underneath. Perhaps the on/off button is not making a reliable physical contact to this switch key board. Perhap if it makes good contact but the switch key board or the wire cable to power switch is flaky.
Power on is first detected by the bios.-- Cheapest thing first, go to Sager's website to look for bios upgrades. Even if there is not corruption in the firmware, the upgrade is a task that should be done eventually anyhow. If it is still not working, follow the trail back from the on/off button. This will take time to disassemble, but may be worth your while.
There is a site that is parting out an 8880 Sager that may have some of these components lefts. They do have 888x disassembly instructions for free download too.http://www.store.yourcoloradosprings.com/sager/
Else you can send it to Sager. I have had little luck with these people and have learned from hard knocks not to waste my time and money with them... Many things can be done by oneself with much greater returns.