What kind of cable are you using to connect the SNES to the arcade monitor? Official PAL SNES SCART cable? Third party Gamecube SCART cable? Direct wiring?
Two things:1. You need to add 220uF series caps on the RGB lines, positive side to the console, negative towards the monitor.2. http://shmups.system11.org/viewtopic.php?f=6&t=28069 - I haven't used one of these, but based off of the feedback this is seems to be the definitive solution for connecting non-JAMMA RGB to an arcade monitor. Alternatively, you could try building your own circuit with a ths7315 (14.5 db gain).