No picture on Mac Power G3

I have a Macintosh Power G3 with a very nice 17 in CRT monitor, running OS 10.2 and OS 9.2

HOWEVER, unless left for (usually) at least a week, the monitor will not be activated on start-up.

ON the occasions that the picture does appear, mostly it allows only one display size — 640 X 480.

Sometimes if restarted immediately it will boot up with the full range of available displays.

The software has been freshly reinstalled to no affect. So where is the problem?