IIRC when I had an O1 and had a similar issue I had to replace the IC on the bumper control board. There's so few parts on the board you could try changing the resistors if they have drifted in value over the years that adjusts the values that the switch triggers at.

Don't forget to look at the simpler things first, like cracked headers.

You can test your vibration theory by hitting the playfield to see if you get similar results as a flipper coil firing.