Check that chain, it can get tangled up on the clip on the solenoid when it slams open. I played around with set bolt on the flapper to take out excess slack. It happened to me once when I 1st got the thing. Temp hit 195, luckily I caught it. Knock on wood, it has been ok since. I had been thinking of rigging a pull rod in leu of the chain, but am too lazy 
Chain can't get tangled now, they have shrink wrap around them to keep that from happening
Hummmmmmm..... If I recall, when mine stuck open, the clip rotate about 180 degrees, hanging the chain up. Can it still do that now with the shrink wrap?
Ok, after eyeball the solenoid set up, if the clip rotates 90 degrees, not 180 degrees because it physically can't, it will hang the flapper open. I was able to get the flapper to hang open about 3/16-1/4" after fiddling with the clip. Then I watched the temp slowly rise. With the clip rotated in the wrong direction, the flapper will not close snugly, even pushing on the it will not close it. So maybe a thru-bolt instead of a clip will work. It seems that the solenoid opens way too violently fast, which can make the clip rotate.
