Yes, my Blonder-Tongue agile modulator developed moving horizontal bars after a couple of years, which condition was cured after I replaced the four power-supply electrolytics. This article has some details:
http://antiqueradio.org/HomeTVTransmitter.htm
Other causes may be at work, but I always like to eliminate the obvious before chasing after the exotic.
Regards,
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html