Random fault on TDA7492's 3.3V embedded VDDS regulator

[SBond.2]

We are using TDA7492 since 2013 on some different design, and up to few months ago everything was ok. But recently we have random faults on VDDS regulator. Currently the percentage of failures, all of the same type, is 5%. Too high for us. In all our projects TDA7492 is connecred almost in the same way, and the heatsink is the same. The only difference is the gain level (pin GAIN0, GAIN1 setting). The fault concerns pin 26 (3.3-V regulator output referred to ground for signal blocks): the output voltage drop to 0.1V ...1.8V instead of 3.3V, and the ampli stops to sound. Replacing the chip the problem disappears. Suppllying 3.3V to pin 26 from an external source, the broken ampli resumes playing normally. On some circuits we performed hard thermal stress tests with temperature probes on thermal pad and heatsink, without encountering any problems. So the questions are: 1) What is the cause of the failure? 2) have GAIN0, GAIN1 internal pull down resistror? From the schematics in ds and in AN3385 it seems yes. 3) It is possible to supply VDDS (pin 26) from any outside source? 4) Is VDDPW (pin 19) input or output? Removing the 22E resistor that connects pin 26 and pin 19, the voltage on pin 19 drops down, so it seems to be an input (in the ds it is declared output). Can you help me? Stefano