Eps is a common failure on these,but it is important that the battery and charging system are operating correctly.There are no user service parts for these ,but there are many firms that do a reman service.I would think it highly unlikely you will get any help from opel,especially on a car this old.

Steering angle/torque sensor is the same unit. The connector from this is grey and can fail, fully or intermittently, giving wrong or zero voltages.

The connector has 6 active pins (8 total, 2 empty) and 3 voltages.
Left hand (turns left when voltage is above 2.5 Volts, range 0.25 - 4.75 volts). When steering wheel is "squeezed" to the left this voltage increases, and the other decreases.
Right hand (turns right when voltage is above 2.5 Volts, range 0.25 - 4.75 volts)
Position (not sure about how it works).
If Left hand or right hand drops to 0 in a glitch, EPS is deactivated.
If left hand has a "neutral" voltage of f.ex 2.2 volts because of established, stable bad connection, turning to the left will be harder than to the right (you have to be above 2.5 volts for anything to happen, and you will never reach 4.75 volts).
Improve the connection with a good contact cleaner, f.ex deoxit, or just move the connector back and forth. Bad connection will degrade this connector very slowly because it is very low ampere, maybe 30-40 milliampere.

Remove pedal cover (2 screws) and you will see the connector.

Disconnecting/Spraying/rubbing the connector various times might necessary to restore connectivity.

This tip has worked for me and some others at valuxhallownersnetwork.

If this tip does not work, the most usual is to buy a reconditioned unit at ebay for those who are not covered by guarantee (10 years/100 000 miles in england), or dismantle steering column and install new torque/steering angle sensor.