1) Never use the power figures from the first run. They will usually be high because the intake manifold (and intake air) is cold. Make several runs, allowing a few minutes between each run. Typically the power levels will settle about runs three and 4.
2) Always allow 10 seconds after starting the car before giving it full throttle. During the first 10 seconds the car is warming up on different fuel and ignition tables.
3) Use an OBD II scan tool to read the intake water temperatures. Try to start each run with the water temperature between 185 and 195 F. To cool off the engine after each run, run the internal heater for about 30 seconds. Switch off the engine, switch the key back on (do not start) and turn on the air conditioning switch. This will activate the aircon fans. Run the fans for a minute or two.
4) The front oxygen sensor on this car is a wideband sensor and can keep the car in closed loop (14.7 air fuel ratio) even at wide open throttle, up to about 4700 rpm. Consider unplugging the front oxygen sensor. This will force the ECU to run in open loop, ignoring any stored fuel trim settings.
5) If you do not unplug the O2 sensor consider running the car in 4th gear and starting the run at very low RPM (about 1000) to give the ECU time to switch to open loop.
6) Consider unplugging the leftmost of the three ECU connectors (above the passenger's feet) after your 3rd or 4th dyno run. This will reset the fuel trim to zero. The ECU will relearn the fuel trim after a few minutes of driving.
7) Fairly obvious but worth repeating. Keep the same wheel and tire and tire pressures every time you visit the dyno. A subtle point here but worth mentioning. Inertial dynos (such as the dynojet) do not measure power: They calculate it. They measure acceleration (F=MA). The acceleration value is used to calculate the accelerating force which in turn is used to calculate the power.
The mass being accelerated is the dynojet roller, the wheels, tires, gear clusters and flywheel. Any changes to the mass being accelerated will change the force and calculated power.