TAZ6 underextruding problem can't solve it

Hello guys, if somebody could help it would be great, I’ve had this problem for 3 months and can’t find the solution.
TAZ6, printing PETG or anything ( same result ).
The printer extrudes too small a quantity of filament and I get this:

It happened one day without having changed anything. I noticed the wheels on the extrusion motor rotate more slowly.
It extrudes less filament, so obviously it boils it, then it does not stick to the bed as there is too much distance, and it is impossible to print anything.
It is not the filament spools, I tried many and they all print perfect in my other printer, a TAZ Workhorse.
It is not the main board, I changed to a new v3 and then to a Chinese v4, same problem.
It is not the extrusion motor, I changed it for a new one, same problem.
I can see no mechanical problem, and there is no particular noise or anything.
I have controlled and very fine tuned the e-steps, I can do it alright the printer seems to work normally when extruding “manually”. But not when printing…
If I set the flow rate much higher ( around 180% ) I can see the rotation speed is more like normal and the filamente gets out with a nice shape, unfortunately although it may print the first layer more or less fine, then the following layers the prints fail, it seems flow is not perfect / not in proportion.
I usually print from Cura, tried other software and same result.

I am a bit lost I must say. Any help appreciated.