I think you will discover that many (at least most) here do not have this kind of equipment for dual monitor operation, etc. to be able to provide any real assistance.
Perhaps if you compare the XF86Conifg file from RedHat that works as desired against the Slack config file, you might locate a difference. As for me I know how to define color depth, resolution and general monitor spec. But how does X determine what vertical refresh rate to use, or how to change it I do not know. Well, I guess you could specifiy the refresh rate is limited to 80-100 and see if that would force it higher since I suppose you want the higher refresh in all modes.
You might check to see if XFree86.org has a mail list you could post the question. I think they do.
Perhaps it has been of a little help.