Welcome Michel.
You might want to check two critical connections.
One - the B+ at the bulkhead jumper terminal (although I suspect this is not the likely issue) - note below as a union splice, and,
Two - the main power connection on the side of the BST at the battery B+. This is VERY suspect in your case (based on symptoms).
The BST connection (to the rear power distribution) is shown here: -
These connections are know to become loose and cause symptoms like yours.