I have a 25k7191 and after about 10 min the pic goes dark you can still see it but its dark
bought a cap kit and flyback off BR and still does the same thing
Buffett would know better than I, and if you have done a cap kit and flyback already, and all of the caps are in correctly, you always have to consider the tube. From my own limited experience (5 machines with dark pictures), they have all been bad tubes. But you were right to start with the fly and caps.