Of course ! The battery !
I bought it in 1999 (or even 1997, I'm not sure) and it had never been changed before (said the seller), which makes more than 15 years of good and loyal service ! Pretty good but it's time for retirement for the battery.
Your explanation seems logical, even more when I backgraded to 1.11 and ram patches were still unavailable.
Thanks again.
Joël