Perhaps check if it the neg popping due to heat. Devise an experiment where you check it the focus after having the light on 1-2 minutes, compare with another check of focusing quickly and then leaving the light off for a few minutes and then checking the focus quickly. If it's due to heat rather than slippage, you can get a Anti-Newton Ring glass from a gepe slide, smooth the edges with sandpaper and place this on top of the negative (inside the neg carrier).