I've been using Debian Lenny on a Toshiba M30X laptop for the past few days, and I was having the time of my life: everything stable, everything working, no headaches.

Today I used it for several hours, then switched it off (shut it down normally). Then in the train I got the laptop out of its bag to show to a friend how Linux looks like, switched it on, and the mouse became all jittery after loading X. Trying to click somewhere caused an impossible mess of vertical and horizontal coloured lines to show on the screen, completely obscuring the desktop. The mouse pointer now looked like a (still jittery) square of different-coloured lines.

I'm now at home, and it does exactly the same when connected to its AC adapter. It boots, everything goes fine, I even get the desktop with the icons and all, but the first click I do the pointer becomes jittery, and a few seconds later it's another jump to ludicrous speed (for those who haven't seen Spaceballs: the coloured lines come up). If it can help diagnosing the issue, the fan always comes on (at medium speed) with the lines, so something's using CPU time. I'd run top and see what that is, but the system becomes unresponsive: even hitting ctrl+alt+f1 has no effect (the weird pointer still moves though).

I can get single user mode working just fine, and I've ruled out hardware issues, so it's gotta be something in video software. I've no idea what the heck's happening though, since I didn't perform any upgrades, installs or anything system-changing at all.

And my friend says she doesn't want "this stuff" on her computer. *sigh*

Edit: what the hell - it works again now! And I didn't do a thing to fix it!
Ok, so it seems to have magically fixed itself - but I need to stop this from happening again. Any ideas?

At home, it works fine.

On the train, using battery power, it starts acting up.

Back at home, after a few minutes on the power-pack/charger, it starts working again. And you've "ruled out" hardware issues?

Yes, I have. It works fine on battery power with other distros, including live ones. In fact, it's working fine right now on battery power on Debian (I've rebooted with the adapter disconnected to test if the lines would reappear).
I can just believe that there's some sort of issue with downclocking of the CPU, or maybe it somehow downclocked the GPU (Radeon 9700 mobility, by the way - no, I'm not running the restricted 3D drivers) and caused it to screw up, but then those wouldn't be hardware issues.

I'm at a loss. It's working just fine now, like nothing happened. I've been using it since yesterday and everything's OK. I just know it'll screw up the next time I want to show someone how great Linux is *sigh*

I've had this happen at one point with an old laptop running a similar ATI video card. I don't know the cause of the problem, but it magically resolves itself. Maybe updating the ATI drivers will help ?

How do I do that? I'm Googling, but all results I can find point me to the fglrx closed driver, which seems to give problems to many people and which I don't need anyway, since I'm not running any sort of 3D application.

If you're using the 'radeon' driver, you'd probably have to upgrade Xorg... not easy.

Ok then... I'll keep it as it is, and if it ever does that again I'll use the fglrx drivers and hope they fix the problem. Thanks.

*sigh*
Running it on battery power definitely has something to do with this. I tried using it in the train again, and it got all slow and jittery again.
I don't think the video drivers can be influenced by the power source. Rather, I think something is getting screwed up in the power management system.
I'm not keen on trying the fglrx drivers since the last time I did they screwed up everything until I managed to boot in text mode and uninstall them; granted that was another distro, but still.

Anyone know how I can fix this? I don't use it very often on the train, but when I do I need it to work.

I upgraded to Squeeze, and it *seems* to have fixed it; I booted once entirely from battery power, and it stayed up. It remains to be seen whether it'll keep working on the move.