I just went through a similar issue on Meteor. Put the game into switch test and make sure each target switch registers with the correct switch number. If that checks out, go back into test mode, hitting the button until 1 is in the credit window. Step through each setting/audit (1-17 I think) by press the test button. If you see trash in the score displays, disjoined numbers like "02 520" clear it using the reset button next to the test button. Restart the game a see if anything changes. I my case I had some bad ram (U8 & U13) that was causing problems. Also, not all of the sounds were there, which is another indicator of bad RAM. I replaced them with some NVRAM (http://pinitech.com/products/stern_mpu200_ram.php , http://nvram.weebly.com/) and it fixed the issue. If the garbage keeps coming back in the audits suspect the RAM.
Another common issue is a PIA failure at U10 and/or U11. An easy way to test is to swap the 2 chips (U10 >U11 and U11>U10) and see if you now have a different problem. If so replace one or both (http://nvram.weebly.com/mc6821p-pia.html).