When you say slideshow do you mean slideshow or are you using the Web gallery output in Bridge or Module in Lightroom?
What size are your images when using this Gallery. If they have to upscale the images this may cause the noise.
Tell us a little more about the workflow leading up to the bridge output.
I use the simpleviewer Flash Gallery in Lightroom (check it out
here) because it makes the images a certain size that doesn't scale with the monitor size. When I scale my monitor up (mine goes up to 1920 x 1080) your images scale as well. This may or may not be an issue.
You can find other Lightroom Web Gallery options
here