If it always comes on at the same time, it should be the timer. I had a similar problem that the sprinkers started working at the midnight even though I didn't program that way. It turned out it was programmed to start at midnight, though I wasn't sure how it got programmed..
If it comes on totally random, I'd suspect the timer is getting some glitch and randomly sending the signal. How about the cutting off the power to the timer and also removing the battery if it has one ?