External monitor not working correctly

Hello all.

My first posting here - from Italy

On my 2002 G4 iMac 700Mhz I have tried for the 1st time to connect an external
monitor via the miniVGA plug, but all I see is a totally flurry and moving
image; moreover, when I connect the miniVGA adaptor, the 'Monitor' control of
the System Preference doesn't 'notice' the adaptor, therefore the Frequency
menu is 'not available' and only 2 of the 3 tabs appear; I have tried to do the
same thing on a 2004 iBook and everything works perfectly.

I am quite sure that there is a problem when I connect the miniVGA adaptor to
the plug; what could this be related to? Can I do something about it?

TIA.
.