Wiring
Hi, you say there are three wires at valve number 5. Two white and one red. This next part is important. Are the two white wires hooked up together or is one white wire connected with the red? If it's connected to the red wire then I would suspect that that one of those two wires is somehow crossed with the master valve wiring. This way valve number 5 would come on anytime ANY station ran. If the red and white are hooked up together try disconnecting one of them and see what happens. Then visa versa. Also, the reason I asked before if your controller was in a metal box is I've seen the symptoms you've described with the funny voltage few times and it always seems to be the same model controller. It's an irritrol I think but I can't remember for sure. It's in a yellow metal cabinet.
Good luck!