You're a very clever guy Brian. I think you pointed out the problem.
I made some test using a lamp to load the power supply at about 3 A. When I connect the lamp, the voltage drops down to 15 V for a while before recovering its rated value (27 V). And when I disconnect the lamp, the voltage then rises (I don't know how much, the power supply voltmeter stops at 30 V) and the overvoltage protection trips. I probably have a voltage regulator that is failing.
Thanks a lot for your help
Jean-Louis