Hi folks,
a quick update in case this can be of interest to others : even after the above modifications, I ran a infinite test loop (a turbo pascal program incrementing an integer and displaying the result) and discovered the display was hanging after around one hour.
I ran a RAM test with no IOs that worked perfectly well. I started to suspect the console IO timing, and indeed that was the issue : i swapped the 244, 32, 04 and 00's for F versions, and the turbo pascal loop is not running since more than 3 hours.
note than i use one one wait state for IO, RAM and ROM with a 10 MHz oscillator.