PKM-25 - if your HI bulb is in good shape, you shouldn't have fall off. I had one for 25 years, before the V54 came along, and when I started having fall off at the edges, I called Aristo (still in business then) and was told that fall off was a symptom of a bulb beginning to fail. I replace it with the V54 (actually I replaced the head with the filter drawer version, still using it today.)
My original research indicated that uniform light is one of the advantages of the cold light. You do have the circular piece of milky plexiglass under the bulb in the head, don't you?
I actually have two diffusion disks in it, one from a spare cold light. I will pull it and see if what you say is happening. At this point considering what I have wrapped up in it, spending $170 in a new bulb would not be a bad thing...
"I'm the freak that shoots film. God bless the freaks!" ~ Mainecoonmaniac ~