That is dramatic. How confident are you that you're conducting this test under the same exact conditions? You mention that you're in an unheated/unconditioned space. For instance, was the original test in the summer after some time messing around on the laser and then your current test was first thing in the morning when it's cool? Did you align or clean your mirrors/lens? Were your numbers really dialed-in? Appreciate you sharing.
But unfortunately doesn't coincide with the previous two statements by Jerome and Bill.