System 3 "Issues"

[jstout66]

I have a 19" System 3 with Space Phone. Love the set, and it has a great picture. However it has developed a problem which I hope doesn't mean I have to retire it, with my limited repair skills.
This morning I noticed it has a solid 3" block that travels vertically up the screen. It's not that noticable, but the "block" is blacker than the rest of the screen, and I seein the center is a bright white dot.
There is no distortion in image as that block moves up the screen. (it reminds me of those channel guide stations) Like I said... the block is about 3" high, and covers the entire screen as it travels vertically.
Is it a cap in the power module? Has anyone seen this issue before?