The under-playfield clicking is the Q and/or T relays, those deal with game over/tilting. It sounds like the game could be in a weird state in between playable, and stuck in tilt. Usually when a System 80 is stuck in tilt, it will not boot, and display strobing 0s on all displays, but your game definitely is booting. I would jumper the MPU to bypass the slam switches, and check your Q and T relays. Make sure the contacts are gapped properly and clean. When you initially turn the machine on, the GI should come on, then 5 seconds later you should hear those relays click once, then the game should go to attract. Is this happening? Also make sure the game has the proper number of balls installed