hi hermanh not sure what the problem could be but I think they are re-badged Telwin which are Italian. is it an AC or DC welder or tig welder maybe stick up a picture of it
the problem sounds strange how long does it weld for before doing this
sounds like it could be a bad connection getting progresivley hotter as you weld
check out all the connections on the rectifier transformer earth clamp and lead inside the unit and just because its a good physical connection doesnt mean it will pass current.
test with a meter if you have one