you can try taking it apart and cleaning the interior. It has a spring-hinged butterfly wing-type plate with a ball bearing. Spray WD-40. If it looks clean and making contact, not likely your I-switch. Connect the two wires going to the switch together (paper clip across contacts) and see if your symptom goes away...if not, the switch is likely good. Disco II, RR classics all have the same switch. I had intermittent problems when going over potholes, etc...reset the I-switch and the 90 ran correctly.
side note: (by previous owner) dealer diagnosed problem as bad fuel pump, after market alarm, bad ECU, all which were replaced under warranty, alarm removed...the dealer was surprised when I clued them in...