Picture skewing

[7"estatdef]

I've been watching my old 6T65 RCA for years now and for the most part it does a great job! Especially with B/W video. However with some of this newfangled color video the picture will skew or twist a bit. Mostly when there is a lot of black and some print on the screen like on movie credits or commercials with large print.
I don't know if this is the nature of the beast or some sort of agc problem as it does straighten out as the picture fades into the next. With the B/W shows they don't seem to have the high contrast pics the the color pics have.
What could cause this?
Terry