Could be the HV rectifier is getting tired, or possibly even the picture tube. Weak video output tube might also cause that. I've never worked on a color set, so there might be other issues. Bad caps are to blame for a lot of things, and if this particular unit uses a voltage doubler arrangement for B+, bad caps in that circuit could cause weak B+.
I'm told that Moyer Electronics is the place to go for flybacks and other oddball parts.
http://www.moyerelectronics.com/