G4 Mac Mini (newly acquired) will not send signal to monitor

Help and advice needed.

For a planned project which involves using Mac OS9/Classic software, I needed a G4 and opted for a Mac Mini (it’s small).

The one I bought on eBay is in great condition, but refuses to send a signal to my monitor. The monitor, a 21in Acer works 100% (on both HDMI and VGA) and is used daily. The Mini has a Radeon 9200 graphics card.

Only once did I get the Mac Mini to connect as it should and for at least 5 minutes I could use it on screen. However, after a few minutes interference (several hundreds of flashing horizontal lines) began, increased, then decreased again to normal. 

Then, in order to install OS Panther (it had Leopard installed, not Classic compatible) I slotted in the install DVD and rebooted, and again it reverted to no picture. I attempted several times with no luck.

I was able to eject the DVD by holding down the left mouse key on reboot, which would indicate the Mini is fine.

I contacted the seller and he says (if I write ’claims’ it might sound as though I am blaming him for selling a dud machine, and I’m not) that Radeons of that era were temperamental and many DVI to HDMI and DVI to VGA cables are decidedly iffy in quality if you get the wrong one (I had bought two new cables). He used an adaptor not a cable and I’ve ordered one of each (DVI/HDMI and DVI/VGA).

Has anyone here any thoughts on the matter? Without a picture my Mini is pointless and useless. I’m hoping one or both of the the adaptor might do the trick but until it arrives, a few suggestions would be welcome.

As I say, I have successfully booted it up and is was working with a picture AOK, but just the once. Looking at the Acer settings, I noticed an on/off switch for DDC/CI (and I have no idea what that does) but I switched that off in case it was a factor with an older, 2005, machine.

Any thoughts?