First thing to check is the power supply +5 volts.
Second thing to check is all in the target head:
Broken tips, cracked segments (usually the pie wedge ones), or a dirty rubber damper holding a switch closed inside the switch matrix. When the game locks up again, does it lock up in the game over (attract) mode or while playing, or both?
If it locks up again during game over (attract mode), unplug the target interface board from the gameboard. Now wait a couple of minutes to see if the game over attract mode resumes. If it does then the problem is in the target head. If it does not, the the problem is either the +5 volts from the power supply is too high or too low, or else the gameboard itself has problems.
If the gameboard is suspected, the problems I used to see on it were either the cold solder joints on the connector header pins (usually on the power input connector) or the video ram chips and/or the video graphics processor chip have gone bad.