My Saab keeps losing power

How can a Saab dealer not find out what the power loss is in my Saab 95 3.0. they took out the onstar and that didn’t help and I’m lost in how to figure this out. the batery is good and checks out. I jumped it the last time and the computer rebooted and it ran for 3 months and then died again and the reboot isn’t working.

Did they check exhaust system back pressure for a restricted catalitic convertor and/or muffler?

Your post is pretty hazy and can be interpreted in several different ways.

Define “losing power”.
Runs fine and stalls goind down the road, sputters and coughs, or as I read this; an electrical fault causing a no-start condition?

And who and exactly how has the battery been determined to be good?
And what about the lighting, etc. in all of this.