I do not have a light bulb limiter but I'm building one this week.
Traveling the next couple of days but I will try the thermal tricks when I get back.
I also have some better test leads coming in for my DMM so that I will feel more comfortable checking things while the power is on.
Tried another experiment today. I might not mean anything but if I leave the amp unplugged over night I can then plug it up and check the voltage on the output and it will be 0V. Then when I turn it on a play for 5 to 6 mins it always stops working.
I tried another experiment today. I plugged it up and checked that there was 0V on the output jack and then left it and didn't turn it on. I came back later which would have been well past the 6 min mark and the 27V was showing on the jack. So it apparently doesn't have to be on to stop working, it just needs to be plugged up.