It's on the 24V motor. I had too many problems with the "bendix" bearings on the original pulleys, so I removed them from the motors. I replaced them with one solid pulley on one motor, and I chose the 24V motor because it fit my needs better. It also fits the 230V motor so I can switch to it if necessary (e.g., if the 24V motor dies).
I doubt very seriously it has anything to do with the motor. The motor is single speed and runs evenly whether the pressure is being maintained or is being erratic. The belt is not slipping during the erratic pressure episode and there is no unusual noise from the motor and high pressure pump. You can see all of this in the video I posted the link to in the original post.
Thanks for your thoughts,