I don't think it's a thyristor problem as, if I get the problem description right, the flash outputs full power also when used in the manual "partialised" modes: 1/2, 1/4 etc. When using those "partialised" manual modes, the thyristor is not involved.

If I had the courage to open such a device I would clean all contacts and check that they contact properly. Maybe there is some "spring shaped" metal stripe on a slider somewhere that must be reshaped.