Classic issues scream of analog board recapping. Mine does the same and I have a bunch of boards and they are all the same. Usually funky video is followed by no sound issue. Also recap of board in the classic is going to be a must. At least with 2 megs of ram in in you know you have the memory expansion board in it. If the plus has more than a meg of ram the resistor mod has already been done so it can be bumped to 4 also.

Gratz on the haul!!