The logic board contains the most vital parts of the Mac (CPU, memory, bluetooth and wifi modules, etc.). If you had the Mac previously repaired with a major issue, they could have replaced the logic board (aka, motherboard) altogether.
Anyway, you should try the hardware reset steps first. Reset the SMC by shutting down the Mac. Press and hold the power button until it seems to power up and shut down again. Press and release the second time to power up normally. See if that alleviated the issue.
If the problem persists, do a parameter RAM reset. Start or restart and immediately press down the command-option-P-R keys and hold until you get the startup chime again but now hold down the Shift key until the startup completes. This puts you in Safe Mode. It takes longer and extensions will not be loaded. Its function is to clear your system caches. Afterwards, restart in normal mode and check again if the issue was resolved.
One thing to be aware of with a PRAM reset is if you have external SSDs connected to Thunderbolt (as SATA), TRIM will be turned off. If this is the case, I can provide you with the steps to reenable TRIM.
I've tried a few things. Shut off the router for a minute and plugged it back in, That seems to have helped. Got the idea from a notice on the TV about not accessing the free movies but should try that there.
But I still had restart problems mostly when waking from sleep mode. I was desperate enough to erase the HD. Not knowing which apps were causing the "restart error message: I researched/interpreted to mean "old or incompatible apps", I've been restoring a few at a time from Time Machine. Damn that's slow..
So far, that means the O/S, safari, AOL mail, my 2 blogs, and Microsoft Office 365 mostly. It restarted twice after that, but has been stable since then. I'll be adding a couple apps per day to see if one causes problems.
I don't have a dedicated Mac keyboard, so I wasn't sure about Options and Command keys. Well, I looked up a Mac keyboard and paper-taped the names on them, LOL!
At the moment, and after several hours today, the Mac hasn't done that random "restart" on me. I will see if restoring some other apps back causes the old problem.
I am having problems restoring photos though. I think they are on iCloud and I am trying to get away from that. I'm trying to get off Google Accounts too. I guess I need to visit ICloud to get my photos. I didn't know they got moved there months ago.
