Could be a failing Shift Inter Lock Unit, this thing:
https://www.ebay.com/i/183393246360?chn=ps
Or a failing Inter Lock solenoid (the thing going click when you put your foot on the brake pedal). I've not had to replace that part, so I'm not sure of the details/part number.
Or some wiring or plug is failing.
First steps would be checking the grounds and 12V lines going to the unit and solenoid.
On my previous SVX I had a problem where I always had to use the shift interlock override button to get our of park all the time. I diagnosed this as a problem with the Shift Lock Unit, spent some time trying to diagnose problem and repair the Unit before giving up and getting another used one. This fixed the problem for a few weeks before it became unreliable. I noticed that pressing on the plug in certain ways made it more reliable. So I replaced the plug (bought one off of ebay and soldered it in), that worked for a few weeks before becoming unreliable again....
At that point I jammed a piece of card board to hold the shift lock button down and just lived with it. I suspect there might be dry solder joints on both my Shift Lock Units, I might be able to fix this by re-flowing them, but I never got around to doing this.
HTHs.
Ian.