no power!

[Haribo!]

well, not enuf anyway. If i turn my headunit to a high volume it turns off. It then turns back on but unless i turn the volume down it will do it again, now once i find the volume i can get away with, if i so much as turn the fans on or open an electic window the headunit will again switch off.  It's been getting worse over time. If my engine isn't running the headunit works fine at any volume. I feel as tho maybe my battery is weak? ? Its a mk2 16v

[Haribo!]

no one got any idea at all? hm, my cars good at producing mysteries

[Ant]

What the amp rating of your alternator?
Have you got a power amped stereo system?
Where the electric windows factory fitted or fitted after?

It sounds to me that you're drawing too much current for your alternator rating, so it can't keep up.

Also check the voltage drop from the alternator to the battery, my leads where broken and cause a 1 volt drop, this was enough to cause a few problems like yours.

There's a post somewhere around here about troubleshooting these problems.

[Haribo!]

amp rating? power amped stero system? i know not of what you are talking.. how do i find out such things? 
my electric windows are factory fitted, i know that much.  but voltage drop? im but a simple man, 

[Ant]

You talk about turning your stereo headunit up, and it cut's out, what sort of stereo set-up do you have, do have a big sub woofer speaker in the boot? If so somewhere you've probably got a amplifier, this pull more power from the battery, which is charged by the alternator.
If you draw more power than your alternator can provide then the stereo could cut out, turning headlights on, powering windows all draw extra power.

There are various guides around on the internet (and this site I think) about checking the voltage across you battery, alternator, etc. I suggest you go have a look, as they'll be able to present the information in a clearer way than I can, you will need a multimeter to check your readings.

Has it always been like this, or at one stage was it all working perfectly?

If it was perfect in the past then either you have a dodgy wire or your alternator brushes are wearing away.

[Haribo!]

oh i see, ive litrally just got the head unit, no amp or sub or anythign fancy like that, yeah it used to work fine, its got worse over time, at the moment it struggles to stay on at alleven with the volume at zero... and very rarely my headlights flick on and off, VERY rarely but its happend once or twice as i turn them on they dont fully come on and then after a second or two they do and then theyre fine, i coast alot down hills and stuff, dont suppose that would have an effect?

[Ant]

I reckon the best thing to do would be to check your alternator brushes, also check your alternator belt for tightness and wear (does it squeal?)

[Haribo!]

nah, doesnt squeal, where are the bushes?

[Ant]

There's different brands of alternator, so I can tell you where mine is.

There's a metal part on the top of the right hand side of the alternator that's in a rounded diamond shape, undo it and remove it, yours should be something similar.
I haven't had to remove mine for a long time, so don't remember if there's any problems or things to be careful (haynes recommends disconnecting the negative battery lead) of so if it falls apart, don't blame me. Best to check a Haynes manual, has a section on removing and checking brush length (minimum says 5mm), check if there equally worn, etc.