Dear all,
Just letting you know that making the autovacuum policy more aggressive seems to have fixed the problem.
It's been 4 days now and everything is running smoothly.
Just a reminder, what I changed was :
autovacuum_vacuum_scale_factor = 0.01
autovacuum_analyze_scale_factor = 0.005
making autovacuum run at 1% instead of 20% (the dafault) and the analyze run at 0,5% instead of 10%.
Maybe it's more aggressive than needed... I will monitor and post back.
Thank you all for your help.