I believe you are on the wrong track.
The function of the OVP relay is to protect the electronic circuits of the injection and ABS computers from excess voltage by shunting the overvoltage by a zener diode, thereby blowing the fuse.
The injection system then goes into failsafe mode, which means it operates by mechanical means only without computer control.
When you pull out the OVP relay, you force it in failsafe mode, and apparently this works fine, which means that the mixture (which is adjusted by the screw inside the tube accessible from the top of the air filter... do not touch it !) is OK and that the injection system in general is OK.
In normal mode, the mixture is fine adjusted by the fuel pressure regulator, which is controlled by the injection computer according to feedback from the oxygen sensor and inputs from air mass and temperature sensors.
So, the problem is most likely within this loop.
You already replaced oxygen sensor and fuel pressure regulator, so the next major point to suspect could be the potentiometer in the air mass sensor.
You can measure the input to the computer by a multimeter. With igntion on but without the engine running, the voltage should vary continously from approx. 0,5V to 4,5V proportional to the position of the air mass sensor plate.
I had a similar problem with flooding (except I could not start at all), and in my case I replaced the fuel pressure regulator (EHA) and thereby ruled this out as the cause of the problem. After trying to find the problem for months, I decided to replace it again, and whoops it worked immediately. So don't rule out the items, you have already replaced.