These are known as the game resetting. Usually caused by power dropping too low especially the 5 volts to the CPU board.
Start by reseating all of the cables. But if the game has not had any work done to the post driver board there are components prone to failure after this much time. Electrolytic capacitors dry out and bridge rectifiers fail.
All of the information is out there since most of the classic Bally / Williams games have eventually experienced this issue. Start with those cables and read about "game resets Bally Williams". Then figure out you comfort lever for doing the work yourself or sending it out.