As far as the RAM SIMMs being a possible cause, wasn't it a common troubleshooting tip way back when to rearrange the SIMMs into different sockets so if there was a trace of corrosion on a pin it wasn't likely to touch the exact same spot on another SIMM. This might be something to try as the sockets could be less than spotless after all this time.

Just remember that the SE/30 uses two banks of RAM; so if you're using two different types of RAM, make sure they are all in the same bank.

Hope this helps (and isn't just an urban legend)

J White