The diode mod doesn't apply to Hydractive-1 systems, as fitted to your car. Ecu H1, H2, H3.
H1-H3 Ecu are normally very reliable.
Have you read the fault codes using a blink Code reader.
See thread
viewtopic.php?t=7368
Does the Hydractive lamp illuminate for 3 seconds when the Ign is switched on and constantly when the Sport switch is selected?
Lack of illumination at Ign on indicates a stored fault code. Reset is by reading all codes to the end of cycle, then hold the Blink Code reader button down for 15s.
Fuse 25 is for the ventillation fan, not Hydractive. Hydractive fuses are F19 (Ign, while engine running), F34 (Batt, for doors open at rest).
For the Hydractive system see the circuit diagrams in the Zip file in this thread.
viewtopic.php?f=8&t=10268
As your car is a 91/92 model year I think it will be using H2 Ecu, with a seperate Gnd feed for the electrovalve.
The wiring diagram linked to is for the H2 Ecu.
As the system functions at all it appears the fuses are feeding power, but the system does need good Gnd, which is mounted on the R/h inner wing, behind the headlamp, and are known to have poor contact due to corrosion on the multipole Gnd connector. Strip, clean, grease.
Test Gnd with a 55w lamp, while wiggling the two yellow wires, to see if it fails under load. Test from the Ecu connector and the 7 Pin connector, even the electrovalve connector if you have ready access underneath.
The electrovalve draws about 4A of initial current, so needs a good Gnd connection.
Does the left display ever show random "Normal Suspension" messages while travelling, if so the Ecu power/Gnd is probably being interrupted and restored.
What is happening is the system starts off working, but once under way and exceeding the 30 kmh activation limit, the Ecu sees a sensor forcing hard mode.
Any sensor fault will be logged in the fault codes, but the accelerator pot could be stuck in the fully depressed position, or the brake pressure switch could be faulty, telling the Ecu the brakes are on, or there could be a wiring short to Gnd.
The Auto-Sport switch forces hard mode on Hydractive-1 system, H1-H3 Ecu.
I have seen the boot lamp switch forcing hard mode after a while as it keeps the door open system operational, while travelling and after the timer has timed out. The switch flap comes dislocated from the trim, so causes the switch to make and break contact when the tailgate is closed.
I doubt this is the case as your car is subsequently responding to a door being opened.
Disconnecting the speed sensor should force soft mode as the Ecu never sees a speed greater than 30 Kmh. Your speed sensor appears to be working, but can be tested with a voltmenter, or oscilloscope, as it is a simple hall effect sensor. (2 wire)
Some people have been known to feed +12v directly to the electrovalve, but I would be wary of this due to it overheating, though I know of a number of Xm that had this "mod" running for years.
The correct operation is +12v for approx 10s, then PWM to give 6v continuous. It is the pwm signal that creates the soft hum one can hear if listening.
The voltage is reduced so the electrovalve is subjected to less heat.
A Dmm, LED, or test lamp is all you need for diagnosing Hydractive.
You can check all inputs at the Ecu plugs, or the brake, body attitude and Electrovalve feed and Gnd, at the 7 pin connector by the R/h sphere.
An LED wired in to the electrovalve feed will allow you to monitor the effect of each sensor while in motion.
If you want detailed information on how the Hydractive system functions, please see the manual in this, linked, thread.
viewtopic.php?f=7&t=10570
Well done on fixing the fan controller.
Which transistors did you use?