OK, I'll change them all.
Maybe the decarb procedure left some residual on the inner part of the sensors.
The front O2's are 40'000 miles old, the rear sensors 85'000 miles.
In the front I have installed original sensors but not exactly for that position. I think I choosed those with the longest wire because at that time I had a set of LT headers and I didn't like the idea of installing extensions.
Maybe not the best idea.
I'll post the results as soon as I'll change them.

Stefano