MESA 5i26+7i76 Startup issue

Hi,
my CNC setup is quite finished and runs fine,
however, I have an recurring issue at startup,
where it takes several attempts to get the motor running at the proper speed , homing is often tricky ...
But every thing works ok if jogged manually. The problem disappears after a successful homing cycle.
What could be the source of this erratic behavior ?
Could this just be a tired PSU, or a deeper problem with my configuration ?
If someone recognize those symptoms, I would welcome any hint...
thx