Thanks, I'll get my oscilloscope back soon and do some more measurements.
I've checked again my MSD, it is properly set with one cylinder selection loop cut and the rev limiter set to 6500.
Checking the documentation, I found a nice feature of the digital 6AL, is the Rev limiter verification.
At each "key-on", it will generate a frequency to display the set rev limit on the tach.
Here below is the video I took of this feature on my car:
https://www.youtube.com/watch?v=CRMfknLT3eU
Seems that the needle sweep to 6500 then stabilize at 5100.
I guess then that there's something wrong inside the tach as my innovate data logger reads properly the RPMs.
Time to open the tach for visual inspection for any burnt component.