Ok, here it is 2 years later and we're still having the same issue. Stalls after 20 minutes. I think I figured out why it'* stalling....
A few weekends ago (when it was still 96 out at 8:30 at night....) I fired up the Park Avenue. Started right up, Always did. About 5 minutes into running, the Check Engine light comes on, but is still running. about 10 minutes later it starts to stumble. I give it a little throttle and it starts hesitating and you can tell he'* not happy. Never died. Since this was the first time I started it at night while trying to figure things out, there was my answer... glowing red hot up against the firewall. The headers were glowing a nice pink color. I do believe I have a clog in the cat, or in the exhaust system someplace. I know the pipes aren't kinked because the car was never raced, or driven over anything, the entire time I've owned it.
So... I'm going to investigate further and find some things out, and write up what I found.
Now, onto the Lesabre'* cluster project post...

Post #1 of this thread was originally created in another thread ( https://www.gmforum.com/buick-172/gr...*-mate-311997/ ) whose newest post is over ten months old. Site rules prohibit posting in a thread that has no posts less that six months old: https://www.gmforum.com/introduction...-first-304557/

Closed that thread and started this thread with this new post.