I have herd before that HDMI converters to component are not very good when extending to more than one monitor, it may be that HDMI is a digital signal being converted to analog whereas VGA to the best of my knowledge is already an analog signal so perhaps retains a stronger signal.
Have you tried the VGA to component converters that are much cheaper than the HDMI converters ?
This is the sort I use:
http://www.monoprice.com/products/pr...seq=1&format=2
It does have a VGA pass through, it may be that you could use that for your singes monitor and the rca out to your RF convertor, I'm not sure on that point but I could try it out for you if you like.
Other than that perhaps if you connect the composite out from your new device to a Splitter amplifier like this it may boost the signal rather than using your mixers video out.
like this one:
http://www.amazon.com/Composite-S-Vi.../dp/B003CMDGJK
I don't know if either of these would solve your problem, I have never tried an RF converter in my setup, just making some suggestions
As to why it only happens on certain songs and not others I can only think perhaps there are more colours in some than others demanding a stronger signal.