One thing you want to make sure of is to pre-heat the oven to 25-30 deg. over the temp you are going to cure at. The reason why is because after you preheat the oven, then open the door, add 15 to 20 to 30 jig heads now you have let out alot of the heat. Its the preheat stage that ruins most fiberguards. The preheat stage is very high heat coming from the elements. If you preheat to 30 deg over your curing temp, when you close the oven door after loading your jigs on the rack the oven does not have to go into the long, high temp preheat mode.
Try this, it works for me.
Mr B