Random Scale Expression

Hey guys- I'm trying to craft an expression that will allow me to wiggle the scale of each layer independently of all the other layers but all connected to the same slider control. I also need the keyframed scale values to remain for each layer. I've tried a dozen different methods but can't seem to get it right.

Ultimately each scale will equal itself + a random number (which has a Max value determined by the slider) I can get that far- but I can't seem to get the random number to occur only every 8 frames or so. Any suggestions? Thanks!