Gottliebs are not exactly the most beginner-friendly pins to repair. Repinning connectors in Gottliebs from the System 1, System 80/A/B era is absolutely necessary. I personally think you should repin all connectors, except for the display connectors as I find those don't really give me any issues. This includes all connectors on the power supply, the interconnect harness, and most plugs on the MPU and driver board. Also, ground mods on every board are a must. I think most of your issues will go away after you do this. When recrimping you will want to reuse the housings as these edge connector housings are unobtainium.

Another thing for these 80Bs, there is a small reset board which you can unplug and throw in the trash (or keep for posterity/parts). It causes more issues than it fixes. Read about it in the link below.

As for your credits dropping to 0, and garbage in the score displays, that sounds like either a RAM issue, or a battery backup issue. I would start by adding some kind of battery solution, whether that be a coin cell battery, a remote battery pack, or NVRam (NVRam being the best, but most costly solution). Then clear the memory as it will get corrupted after adding your battery solution. Does your board have any alkaline corrosion damage? That could also be causing weird game issues.

Relevant links:
>Crimps for the white connectors on MPU and driver - http://www.pbresource.com/molex.html (part number GTB-0850-0105)
>Crimps for the black connectors - https://www.arcadepartsandrepair.com/store/pinball-kits-parts/pinball-parts/bifurcated-edge-connector-split-pins-18-24-awg-100-pack-me1004/ or alternatively https://www.docentelectronics.com/httpdocs/AmusementGames/Pinball/Gottlieb/system%2080%20AB.htm
>There are a couple traditional connectors on the power supply which use standard .156" crimps, not the Gottlieb bullshit. That would be these and can be commonly found in lots of places - https://www.arcadepartsandrepair.com/store/pinball-kits-parts/pinball-parts/molex-156-trifurcon-pins-female-22-26-awg-loose-100-pack-ca1008/
>You will need a pin remover to get the crimps out of the housings, which Docent Electronics also sells and which comes with the edge connector kit (note that the edge connector kit they sell comes with pre-crimped wires, and doesn't include the black housings
>Guide on repinning -

>Ground mods - https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Ground_updates
>Reset board - https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Reset_Board
https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80#System_80A_.2F_80B_Resets

>NVRam - https://pinballelectronics.com/index.php/product/5101-nvram-battery-eliminator-module-for-gottlieb-pinball-machines/

A couple other things...

The 80B MPU has a daughter board which commonly gets bad solder joints causing the game to lockup or not boot. If you find this happening consider getting this replacement board which bypasses the original daughter board. This way you don't have to worry about removing the old, and you can simply use the new.
https://www.pinitech.com/products/gottlieb_piggydeux.php

Check out the suggested repairs for the 80B power supply, it's fairly simple stuff. The 80/80A power supply rebuild is way more involved.
https://www.pinwiki.com/wiki/index.php?title=Gottlieb_System_80#Recommended_Repairs_for_the_System_80B_Power_Supply_Board