Formac Monitor G5 Incompatibilty


When I recently bought my new G5 power mac, I was assured that it would work fine with my old 21" Formac monitor. However, the two seem to be incompatible. The monitor works fine with my old G3 and with a PC - the G5 works with two other monitors a CRT 17" and LCD 19". I took the monitor and Mac to an Apple repair company, they said my monitor was too old - something to do with refresh rate and resolution. Has anyone had this problem, is there a work around or do I have to fork out for a new screen?
My graphics card is an ATI Radeon 9600.