My girlfriend was mowing with John Deere 425 yesterday and texted me to say it started running real bad. Then she said the temperature gauge was high. When I asked her if it was in the red she said it may have been in the red but not for long... of course, I don't know if she looked at the gauge before it started running bad. She said she shut off the mower right away then drove a short distance to garage and shut it down completely. When I got there, I noticed the anti-freeze was way down. I didn't notice it leaking anti-freeze or any other leaks. I added quite a bit of anti-freeze then parked it in the garage. When I was backing it in, it started running real rough like one cylinder wasn't working. Today, I took off the side panels, and when I took off the side that was running real bad, something fell onto the ground. It was a spark plug. The center of the spark plug was loose, I could slide the porcelain center up and down. I got a pair of new plugs and started it up. It seemed to run faster than it should and doesn't sound right. I didn't run it long enough to see if the anti-freeze went down. I have a compression tester. If I test the compression what should it show on each cylinder if one of the head gaskets is bad? Does this sound like a blown head gasket? I didn't notice any excessive amounts of white or blue smoke, nor did I smell anything weird. But as I said, I didn't run it very long. Any thoughts would be appreciated. If its a blown head gasket is that a pretty straightforward job on this garden tractor? Should I do both at the same time?