Steps to revive a flagging D-Jet car.

1) Tune up EVERYTHING else first. Timing, valves, etc.

2) Diagnostics on the D-Jet.. A multimeter used to check readings at the computers multiplug is best - that way you check the wiring and sensor at the same time. http://www.icbm.org/erkson/ttt/engine/fuel_injection/d-jet.html
Luckily the system is pretty simple and there aren't many sensors to check. And the intake air sensor really doesn't do much in any case - the coolant sensor really makes the engine run rich, as can the manifold pressure sensor. The car can still run, however, with all those sensors disconected...

3) If you have a MODIFIED engine (i.e. more displacement) the properly tuned D-Jet system will run lean. It has a static model of the stock B20 engine and how it breathes throughout the RPM range in conjunction with the manifold pressure. Changes in displacement greatly effect this model, changes to breathing patterns due to cams and porting and exhaust work affect it to lesser degrees. So a properly working D-Jet system is not appropriate on all engines. The little dial on the computer is NOT the way to fix this - all that does is adjust the mixture at idel (a special idle circuit is triggered by a set of contacts in the throttle switch). At the factory each car was individually hand tuned after completion with a exhaust sniffer. This individual hand tuning made them clean enough to pass emissions laws without needing all the extra junk many 70's cars had. But off idle it ignores that setting. To adjsut the mixture off idle you need to trick the system into injecting more fuel. One way is to modify the coolant temp sensor readings going into the computer, as already suggested. But I find it easier to raise or lower the fuel pressure slightly - there is already a built in adjustment and it affects mixture in all operating conditions. Perhaps the best way is through trial and error - run it hard, pull a plug and inspect. Richen it until you see a tiny bit of soot, then back off slightly. Access to a exhaust sniffer and a rolling dyno would obviously be perfect for this. Nearly as good would be installing a fuel-air mixture gauge that utilizes an 02 sensor.