I wasn't aware it was a common fault, I thought I just had a dodgy stereo. Mine doesn't turn itself on but the volume control is crazy, sometimes up is actually up but then it'll change and can take a few minutes just to get it to the right volume. Move it a bit too fast and bang, your down to volume level 0 and takes an age to get it up again.
 
spray a little wd40 into the volume knob and twiddle it but hold some tissue underneath

this has always sorted mine out ;)
 
Trust me - non e of these fix the problem. It's always gonna be like that. The best "fix" we did was to put a large blob of blutak on the knob and press half a cocktail stick into it so that you have 2 bits of stick protruding from the know. Looks naff but makes it easier to adjust the volume. Still a problem - esp. if you go over a bump while trying to adjust the volume
 
If it's the same as mine where the stereo and heater switches e.t.c are all built in and it has the display screen in the middle then all you have to do is leave a cassette tape in. It's deffenitely a common fault. I had the same problem when I first got my car, did a bit of googling and hey presto! Problem sorted.
Turned out there was a lot of people with the same annoying problem. It's bonkers but it works.
 
I always have a cassette left in there as I have a cassette to iPhone connector for my music, but my messed up volume control is still present. Perhaps you just got lucky?
 
I've left it off and just extended the speaker wires and fitted a better stereo in the shelf above the drivers feet. All you need to do is take the live straight from the battery and find a good earth. It's a far better sounding stereo and it's got the outputs for my bass box. I'd like to find a way to cut the original out and put my stereo in its rightful place though but it's flippin tricky.
 
Is this to do with dust getting behind the volume knob I wonder?

I had an old stereo that started doing the same and sometimes when you turned the volume knob really quickly in alternate directions it would appease it for a while. My stereo has just started doing this with the volume, but very intermittently.