The suspension can do all manner of strange things and what you describe might well be normal depending on circumstances.
Going up is quite possible. Say four people get out and the car is lighter and will rise. Shoud the anti-sink valves snap shut before the height correctors have a chance to do their thing, it will stay high.
Parking up with the front wheels at near full-lock will cause the front to rise.
Even with anti-sink valves, it is not unusual for the car to sink sometimes and stay up at other times. It all depends on how it is feeling. Sometimes just leaning on it or popping something weighty in the boot will make it sink. Also it might sink/rise if parked on a slope.
Have you noticed yet how it sinks as you fill it up with fuel? If you lean on the car it feels mighty funny and even funnier if the accumulator is in good nick and it corrects itself whilst you are still filling
Groaning is almost invariably caused by the front struts needing lubrication on their rams. Set the suspension on high, pull down the strut gaiters (not easy) and apply thin grease to the exposed shiny strut pistons. There is no need to do the rear struts as they are self-lubricated by LHM leakage; the front struts are the only part of the hydraulic system not lubricated by working leakage.
I'd give the height corrector linkages a good clean and lubricate to rule out any stiffness there causing a problem and then test the efficiency of the accumulator sphere by switching off and then immediately sitting on the boot sill. The car will sink and if the accumulator is good, it'll rise again using pressure stored in the accumulator. A good accumulator will do this twice.
If it won't rise on this test, the accumulator is weak and should be replaced. This'll go a long way to resolving it's other behaviour too. A weak accumulator will cause the anti-sink valves to shut much sooner than a good, healthy acumulator so height corrections triggered by passengers alighting cannot take place.
Granny suck eggs time: When cleaning and lubricating the height corrector linkages, you need to get right under the car. NEVER venture under a hydraulic Citroen unless it well supported on axle stands, over a pit or on a ramp. They can suddenly sink (especially when working around height correctors) and crush/kill you. Take great care.
