Here's a puzzling situation. About a month ago we did a repair where we swaped the thermostat module for a Peerless range (Gas only design) after their report that it was both not consistently getting up to temperature, plus the range pilots wouldn't light. The replacement fixed the pilots, and when we did a test fire, the oven hit the target temp of 350 in seven minutes.
However we were just informed that a few days ago when they fired it up with the target temp of 400, the oven took exceptionally long to start firing, and didn't reach 400 for nearly 40 minutes.
Looking at the discrepency in timing, the only think I can think that we may have done differently is cracking open the broiler tray during the test firing (To better hear the oven firing.)
Any advice on what I can do to try and figure this out?

Thanks