many thanks for the RESET line rubber_jonnie, you nailed it !
I've tried C100=47µF and C102=22µF temporarily before the complete motherboard recap and it now boots as expected
I've also changed CPU to MC68HC000 and everything performs well.
Next mods : full motherboard recap with Pana FR, 4Mo RAM, floppy disk to 1.44Mo, audio DAC mod, video ghosting mod, resistor array check.
Cheers
A.