Because new PSU is likely fraction higher voltage than original , it does not take much to upset these old machines. I suggest you try soldering 10uF across C1 to see if that fixes the problem. Also could try 10uF across C7. It will add more delay to the reset circuit which should solve the issue. I see the same problem with STE machines.
Chris

donc voici la réponse de Chris, (je lui faisais part du fait que je n'avais pas le probléme avec l'ancienne alimentation). il ne reste plus qu' a trouvé ces "10uF"