The 23c series has a couple of delrin cogs that control the hight adjustment and those go over time, but I haven't had the focus shift on our (12) 23c's in a dozen years or so of working with them. If the proper 75 watt bulb is in the top, I have not found much of a problem with negative popping either. In fact, my students are often anything but prompt in their process and they still have few problems with heat. These are somewhat elderly mechanical beasts, so my first thought is that some nut somewhere needs snugging up a bit.