Is the RSX FD made for a double or a triple crank (they were available both ways)? Double crank RX100 FDs were/are more common. The bottom bracket would have been changed to work with the triple crank, and maybe the bottom bracket that was installed is on the wide side. do you have much clearance between the crankarms and the chainstays? If so, you may be able to get a narrower bottom bracket that would be more compatible with your RX100 crankset.
You could also consider switching to a mountain bike triple FD. Many/most road bikes that came with triples at that time used mountain FDs because of the lack of suitable triple road FDs.