Another hint:

When the fuel trims are not acting weird they are at idle (cell 19) somewhere between 0 and + 2%. STFT are pretty close to zero.
If I disconnect the MAF (engine still running) the fuel trims aren't really changing.

When the engine is hot and the fuel trims are negative (as described in the beginning of this post) and I disconnect the MAF the fuel trims go back to 0-2%. I reconnect the MAF and they go negative again.

It looks like the VE is commanding a pretty correct quantity of fuel.
But I can't imagine what causes the sudden negative change of the fuel trims.
I can't imagine that the MAF suddenly is measuring 7-8% more air.
I was thinling the MAF was getting too much heat because of the new location (Vortech setup) and I made an insulation. It didn't help.

It's the MAF? What else can change with temperature?

It's a sudden change, not something that increases constantly with temperature or some other signal... when the fuel trims make a jump it takes few seconds!