Problems Starting Gravely Mower?

I have a Gravely MiniZT 1534 mower. It's about 8 years old and has worked well until yesterday. I went to start it and it acted like the brake wasn't on (ie., nothing happened when I turned the key). The brake was on, so I pushed the brake lever off then pulled it on again and turned the key and it started fine. It worked for about an hour until all of a sudden the blades turned off while in motion. I stopped and turned off the mower to check for a snapped belt or broken pulley, but everything looked fine so I went to turn the mower back on and nothing happened. I tried the brake (off then back on), but no effect. As I listened closely, I could hear one of the two relays whistle-humming while the key was in any position except off. Normally, a noisy relay equals a bad relay, but I bought a new one today and the mower is doing the same thing. The mower had two relays, both the same, so I even swapped the new one into the other position to check, but no effect, just the one relay is humming when the key is in any position except off.