I signed up specifically to thank [USER="142084"]WBA[/USER] and @Wilderone: I had the same problem and thanks to their comments, I checked found two cold solder joints on one of the transformers.

Seems to me that the boards are a little thin and flexible, and since the transformers are rigid enough to put stress on the joints as the board flexes around them, the weakest point is the joint. Presumably the flexing is further exacerbated by heat.WBA...