Overhead garage door opening on its own
The fact that it's intermittent makes it hard to prove. I suspect the manual switches you refer to are the type with a separate up button and down button rather than a single button type control. If that is the case then I suspect the up button (or the wires from it) has made a connection (short). That would explain it not working when you push the down button because the up is still in effect pushed. It could also be as you say a short in the board giving the signal to open. Some openers have a push button on the board also. I would try disconnecting the wires at the opener going to the manual controls. If it works for a month or two without any problems then I would replace the switches.