does 768 meg ram cause conflict?

I had to buy an older computer to accomodate an ISA card, I bought the top specs available with ISA (Celeron 1.3) Everything seamed okay (the ISA card is an interface to a motion controller driving a computerised engraving machine) but every so often the PC just closes down and then re-boots, I was advised that the PC might be too fast for the software etc., anyway the seller of this PC advised to remove one of the ram cards, open opening i notice that there were 2, 1@516, 1@256, I’ve removed the 256 and it seams to be working (although I cannot guarantee until I keep it on for a few days).
768 Ram seems unusual? Does this make sense? and could be the cause of it shutting down?

Any help please,
Thanx-David