The electrocardiogram of impossible diagnosis: Diffuse biphasic up-down T-wave

This ECG belongs to a middle aged man, asymptomatic, hypertensive well controlled. It was repeated several times and always kept the same ECG pattern.
We are sure that it is not electrode misplaced, or loose.
Echocardiogram: normal.

Question:

How to explain the biphasic «plus-minus» or up-down T-waves diffusely registered?