@COsgood: if you have two valves open at the same time when sequential is checked, I assume it’s either because one is a master valve, or is defective and cannot close once opened. in the latter case, the defective valve (most likely shorting) will start drawing too much current that causes the voltage from the 24VAC transformer to significantly drop, hence the controller crashes.
As mrburns42 said, adding MOV would not help in this case because the problem is not caused by power surge.