If you have extra 5101 RAM you could try that in the board. It does sounds like it could be bad memory. Unfortunately in order to try a different 5101, you need to remove the existing RAM that's soldered to the MPU & install an IC socket (but you would need to do that for an nvram upgrade anyway).
I should have an Gottlieb Sys1 NVRAM option available again later this year. I actually had a module available last year, for a bit, from Pinsider @cactusjack who had some extra boards made up from when he upgraded some machines. The board was pretty large, so I shrank it down to about the size of the 5101 nvram I sell. I just need to send it off on my next PCB order (which should be happening relatively soon). At this point, I'd expect March/April time-frame I may have some of those for sale again (only in the *NEW* compact design). Just wanted to let you know that was coming..
https://www.pinitech.com/products/gottlieb_sys1_nvram.php (out-of-stock currently, just a link from what was being sold last year)
http://www.pinitech.com - "Pinball Inspired Technology"
NVRAM, Bally/Stern LED Displays & Mods for pinball machines