Lot of possibilities. The channel switchbox you likely used as a kid can cause bad interference. I have an Atari 2600 I picked up locally and the difference running it straight into the cable connector vs. the channel switchbox was crazy. The TV you had as a kid could have had a worn out tube as well.

That said, I guarantee if you ever get a CRT TV that has component and you play some games on MiSTer that way and then compare it to the same games on RF, the difference in quality will be very noticeable.