It was useful to learn that increased airflow results in increased load for the type of blower we have. The suggestion to look for leaks was a good one.
I found a sizable leak as shown in this video.
Shared album - brad clements - Google Photos
The 'air chest' is in fact a room about 9x6x8'. The entire back wall forms a bellows. The wall is spring loaded. This video shows one corner of the bellows has been torn open, the black metal bar is one of the springs.
If there wasn't a hole here, the wall would move outward about 8" to compress the springs. That movement would pull a bar inside the air chest that would then close the intake valve from the blower.
When we get this repaired I will follow up in this thread with new current readings.
Thank you to everyone who commented on this thread!
The automotive repair people use a smoke maker to look for leaks around window & doors.