First Try taking the OVP out and seeing if it makes a difference..
My ABS recently went mental, and to disable it I ran the car without the OVP relay in. (Discovered now this isn't a good idea for long periods). It was hard to start when cold, and would stall until warm, then ran fine. I assume these are the symptoms of a broken OVP, and they don't really sound like yours..
If your car runs fine for 3-6 seconds, and then *suddenly* switches to rough idle, it's likely to be electrics/spark rather than fuel.. I would get a tech to diagnose what it may be.. and then decide how you wanna go about fixing it, rather than replacing random bits & bobs ;-) .
2.5-16v 1990 90,000m Astral Silver
2.0E 8v 1986 107,000m Black