Ours was the ECU
When we had the same problem, we traced the problem to the airbag ECU, and had to replace that. This was done by swapping the ECU from one of our 156s to the other, and the fault moved with it. We had to get a new ECU, which was NOT cheap.
(The ECU is behind the centre console with lots of yellow wiring, if it does get to that stage with yours.)
1963 Giulia 101 Spider (RHD)
1992 75 2.0 TS LE
2001 916 series Spider TS Lusso
2000 156 2.0 Lusso Sportwagon