Could be tubes waking up after being dormant for 50 years, resistors drying out and so forth.
I pulled my set out a few weeks ago and ran it and it looked better than it did a couple years ago when I last fired it up. My CRT is a little slow but it came right up this time and was razor sharp.
I made a short video of it:
http://youtu.be/DkBcYmZVUkk
These sets have a much better picture than any of the 7JP4 based sets I've seen.