Unless it explicitly says "out of memory", I would not expect RAM to be an issue. How much RAM do you have?
As for why it is keeps crashing, I don't know. I rarely see SSMS dying myself, but SSMS is a versatile tool, so you and I may not be using SSMS in the same way. That is, you may be hitting a bug in some corner of SSMS that I don't touch myself.
I don't want to point fingers as Red Gate, but that is certainly a suspect.
You could try SSMS 19 Preview 3 and see if that works better. Note that installing SSMS 19 will not affect your current installation, but you can run SSMS 18 and SSMS 19 in parallel.