you're getting ram errors because the voltage is probably too low. in my travels I've found Joust for whatever reason to be the most prone to issues stemming from low voltage, despite the fact it's otherwise identical to Robotron (which cooperates well) and Sinistar (which can be a pain in the arse)
there's 4 capacitors nearest the power header on the MPU. each carries a voltage: +5, +12, -5 .. the final axial cap is for ground. the first 3 should be 100 uf / 16V stock, and the axial 22 uf / 16V. you can just read them too.

these caps are ancient now and you will lose power through them. the rom board should have 2 of the 100 uf / 16V caps on it as well -- I personally replace these with 100 uf / 25V.
replacing the headers is also particularly important, because the factory ones weren't that great to begin with.
if you know how to crimp molex, I also encourage replacing the plug to the MPU. if you're using a linear power supply (I imagine you are) you should also change the plug(s) to the outgoing DC header.
if you don't feel confident you can resurrect this, I now am offering repair services which cover MPU, rom, and power supply boards. if you want extra diagnostic on the input board to make sure the ribbon is good, I can check that as well.