If it were a software issue then I guess it would affect anyone using a 7300 GT in that mode. I would have thought there would be some reports on the Apple Discussions forums if this were the case. I don't see any significant volume of such reports, which suggests it's more likely to be a hardware issue.

In reply to a post by AlanH:

which suggests it's more likely to be a hardware issue.

Just what I needed

I was browsing the system log (as one does ), and there's a lot of very strange-looking errors coming up in there. Far too many to include in a post, but a few examples, the last two of which are generated several times::

May 21 12:51:01 bill-fords-imac mds[35]: (/Volumes/Data 2/.Spotlight-V100/Store-V1/Stores/9710DC80-C440-410E-82F4-40D0991D3DFB)(Error) IndexCI in ContentIndexOpenBulk:Could not open /Volumes/Data 2/.Spotlight-V100/Store-V1/Stores/9710DC80-C440-410E-82F4-40D0991D3DFB/live.2.; needs recovery

(That one turned up on the forced restart, on all three external drives)

May 21 13:01:46 bill-fords-imac [0x0-0x23023].com.apple.systempreferences[239]: objc[239]: Class O3Panel is implemented in both /System/Library/MonitorPanels/AppleDisplay.monitorPanels/Contents/Resources/Authorization.monitorPanel/Contents/MacOS/Authorization and /System/Library/MonitorPanels/AppleDisplay.monitorPanels/Contents/Resources/TVOptions.monitorPanel/Contents/MacOS/TVOptions. Using implementation from /System/Library/MonitorPanels/AppleDisplay.monitorPanels/Contents/Resources/TVOptions.monitorPanel/Contents/MacOS/TVOptions.

and

May 21 13:57:13 bill-fords-imac /System/Library/CoreServices/backupd[321]: Node requires deep traversal:/Volumes/Data 1 reason:kFSEDBEventFlagMustScanSubDirs|kFSEDBEventFlagReasonEventDBUntrustable|

Disk Utility finds no problems... could something have got corrupted (re-install?) or should I start trying to find my nearest Genius?

My suggestion that it's hardware assumed that your software is the same as everyone else's. But if there is some general system software corruption then the display issue could possibly be related.

An archive and restore, using your system software DVD, may well be worth trying. It should leave your existing user data and prefs unaltered but replace your system software.

In reply to a post by AlanH:

An archive and restore, using your system software DVD, may well be worth trying. It should leave your existing user data and prefs unaltered but replace your system software.

I've seen this referred to before, is it an option if I boot up from the install DVD or is the archiving something I need to do first?

My install DVD is the original 10.5 Leopard, so I'd better allow plenty of time and bandwidth for Software Update to do it's thing

It's an option in the installer. Watch out for an Options button bottom left as you run through the installer screens.

The other options are erase and install, which reformats your hard drive first, or update, which will probably not replace existing faulty system files.

Thanks Alan, I'll probably give it a try tomorrow.

A&I is the way to go - don't forget to check "Preserve user settings"

Well, that seemed to go more-or-less OK. Only problem was that it decided it didn't want to know about the admin password after the restart, so I had to reboot from the DVD and re-set it, which led to one or two access problems. They seem to be sorted out now though.

Updating everything to 10.5.7 took a while though, getting on for a GB of download

I'll leave the screensaver off for a while to make sure everything is OK, then enable it again and cross everything I've got two of that will cross

One query- can I get rid of the "Previous Systems" folder without doing any harm?

I'm never sure what that "Previous Systems" folder is good for. I've certainly never had cause to go into it, and I'd be very reluctant to start moving bits and pieces from it into the live system.

Roughly what I thought

OK thanks, I'll wait until Time Machine has had a crack at it then delete it.