You have new injectors, so presumably none of the 4 cylinder injectors are leaking. Based upon the condition of the plugs, are all four cylinders running rich?
Make sure the cold start injector is not leaking. Also, the cold start injector relay is normally only energized when the starter motor is engaged; however, if the cold start relay sticks it is possible for the injector to remain continuously energized. This is rare; but, easy to check by disconnecting the electrical plug on the cold start injector - if that improves running you know what the problem is.
As suggested by others, check the resistance of both the coolant temperature sensor and the inlet air temperature sensor and confirm that they are within range for the temperature as specified in the manual. Also, check the condition of the wiring at the plugs for both sensors. The wiring becomes brittle with age and may short or be broken.
As Planetman suggests, check for vacuum leaks on the MAP sensor. In addition, you can check the condition of the windings in the MAP sensor using an ohmmeter - the specs are in the service manual. I think winding failures are pretty rare.
Check the condition of all the D jet wiring, particularly at the terminal plugs. Wiring / connector issues are the bane of ageing fuel injection systems.
I don't think its the cause of your problems; but, how have you set up idle speed? Did you follow the procedure in the manual using the idle air by-pass screw? If somebody tried to adjust idle speed using the throttle stop screw, it will mess up the setting of the throttle switch which just complicates things.
On the box containing the D jet controller, there is an adjustment screw for adjusting CO levels. Any chance somebody was fiddling with that screw?
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