When you say rise up and drop down again, do you mean it will keep doing this indefinately in a cycle - up, down, up, down all by itself ?
If so there are number of possible causes.
First of all, after it finishes rising, does it immediately start to sink slowly, or is there a 5 to 10 second delay with no movement, and then it drops quickly ?
If it immediately starts to sink slowly after it reaches the peak height, there may be internal leakage in the front suspension somewhere - struts or height corrector. Not very likely, but I had to mention it for completeness sake.
If on the other hand there is a 5 - 10 second delay and then it drops quite suddenly (and then repeats the cycle) it is due to mechanical stiffness (high breakout friction) in the movement of the suspension - most likely the front struts need lubricating.
Doing so is a DIY job, but you'll need safety axle stands and a bit of care to do it safely, the method of doing it has been discussed at length on the forum a few times before so a search should turn something up.
What is happening is this - height corrector thinks the height is too low so starts admiting fluid into the suspension. Normally this would cause the suspension to rise smoothly and when the correct height is reached it stops, and everything is fine.
When the movement of the suspension has high breakout friction, as more fluid goes into the suspension and pressure builds up, no movement occurs for quite some time, eventually the pressure will overcome the friction and the suspension will lurch upwards, but will come to rest too high.
Now the reverse will happen. After about a 5 second delay the height corrector will start letting out fluid but nothing will happen until the friction is overcome and the car will lurch downwards, and the cycle will repeat.
Amusing to watch, but somewhat annoying if you're trying to use the car as a car...
There are two circumstances where this behaviour can happen on a car that has struts which are in good condition - one is on full steering lock, and the other is if the car has been driven forward or backward with the suspension right down or right up.
Due to the (poor in my opinion) steering geometry in the Xantia, full steering lock applies a lot of side thrust to the strut bushes, and this can temporarily increase the breakout friction enough to cause this oscillating behaviour.
Also, the full travel of the suspension geometry causes some sideways movement of the wheels, and if the car is stationary the side thrust generated can also cause this behaviour - for example if you lower the suspension, roll the car forward a bit, then set it to normal height, most Xantia's will start oscillating up and down like this.
Another time this will happen is if for example you lift the suspension fully up, then jack up the front of the car for some reason lifting the wheels off the ground, then lower it back to the ground, and then set the ride height to normal. It will then almost certainly start oscillating, because of sideways thrust on the struts.
The "cure" in both cases is to simply roll the car slightly while near the normal ride height and the stress will be released, and the oscillating will stop.
So in summary, make sure it isn't just doing it on full steering lock (which is not uncommon) also when at normal height try rolling the car forward slightly, if it continues to oscillate by itself the struts need lubricating.
Regards,
Simon
