Since your car isn't (I assume) US spec, I have to make a few assumptions, mainly I assume this is a SFI (sequential fuel injection) engine, probably LH or HFM-SFI, both really the same system, only the design of the mass airflow sensor is different.
If this is the case, I would start to suspect the engine wiring harness is the problem. There are a number of places this can be checked, although some of the areas may not apply to a Euro spec engine. The places I can usually make a good confirmation on this problem are the front of the cylinder head. The plastic panel needs to be removed, and there may be a couple switch-over valves under that. There will be electrical connectors on these switch-over valves. There are conduits over the wires for these connectors, sometimes you can see the insulation cracking off the copper wires just by observation of the wires coming out of the conduit, if not it may be adviseable to cut back the conduit to see more of the wiring. What happens is the insulation gets brittle and flakes off of the copper wires. If this happens in places where the copper of different wires can touch, you can have various problems, including the symptoms you have described, although I have to admit that by your description, it really sounds like the overvoltage protection relay, but you've done that already. Other places to check for a failure of the harness is the harness for the mass airflow sensor (harness outer covering needs to be cut open though) and the individual wiring connectors for the injectors. On a US car, usually you will get a "check engine" light, but a Euro-spec car I know has different equipment in this regard. Maybe there are stored codes in the engine management system similar to a US-spec car which may indicate the failure going on, even though there isn't a "check engine" light like a US car?
That's the best advice I can give assuming it is a SFI engine.
If it isn't SFI, if it's a CIS-E engine, let me know. If it's carburetted, you're on your own!