I have done battle with the D-Jet many times (I will be retiring my system and replacing it with SU carbs this weekend). In this case I would check what you have already checked, then I would move on to the throttle box (that gadget located on one side of the air intake side of the intake manifold). This is very likely to go bad since it is a contact/slide switch (whatever they're called, *variable resistors*). They get dirty like the knobs on your radio and the same approach to fix the latter will fix the former.

Get yourself a can of tuner bath (Radio Shack or better[I hate those bastards...don't give 'em your address]) Remove the throttle box, then prise open (flathead screwdriver will do) Inspect for crud and scoring along the path of the moving contact (moves with your gas pedal) If it is worn through anywhere, you need to replace the unit(junkyard will do). Bath all things copper and scrub if you need with cloth and fingernail. Put it together and think good thoughts.

One more thing: I have read that the vacuum advance can get old and do all kinds of strange things. Take the tube going across from the manifold to the advance unit on the distributor and tie a knot in the middle (or remove it and plug at least the crucial one of the two exposed holes). Some may argue with me here, but it does nothing to the behavior of your car, but insure that your old, crusty vacuum advance isn't screwing things up. Also, this may be the source of a vacuum leak since the diaphragms tend to develop holes, especially when the car has run poorly (i.e., has backfired a lot). The vacuum advance is more trouble than it's worth, really.

Also, if you want to give up on the FI setup, I know a guy who has many SU carbs with manifolds to boot. These are the most dependable fuel delivery systems on the old models. I will never again travel long-distance with a D-Jet setup. Too much troubleshooting involved with them.

Regards and Good Luck, -jarrod