I'd rather not guess and lead you down the wrong path (especially with Philip reading this thread..

but from what you describe, it starts juddering when the ECM enters closed-loop mode, so:
I would concentrate on the Engine Coolant Temp sensor (ECT) connector,
follow the top radiator hose to the motor, look for the (2) wire connector on the right side of the thermostat housing,
Check the (gauge) coolant temp sensor on the left side of the thermostat housing,
Check the Mass Airflow sensor (MAF) connector, located in-line with the flexible rubber tubing between the airbox assembly -> leading to the top of the motor,
Check the Air Intake Temp sensor (IAT), mounted in the top of the airbox cover (the air filter is underneath),
Check the upper O2 sensor (HO2S) connector, located on the right side of motor (on US models), connector is mounted parallel to the exhaust manifold: just follow the wiring from the upper O2 sensor -> back to the connector,
Check the Crank Position (CPK) sensor connector, located in the bell housing, behind & parallel to the exhaust manifold: follow the wiring from the sensor back to the connector,
Check the Cam Position sensor connector (CPS), located at the back of the motor, center: the CPS sensor is mounted on the (Exhaust) cam (on same side as the exhaust manifold, back of valve cover): follow the wiring from the sensor back to the connector,
Check the Ignition coil lead, located at the back of the motor, left (Intake) side: if necessary, remove the top metal intake tube leading the the throttle body, remove the plastic cover, inspect the wiring lead from the coil packs and follow the wiring from the coil packs -> back to the connector,
If you -really- doused the motor, then I would go through the step of removing the coil packs and inspecting the spark plug boots & wiring as well,
I would use an alcohol-based electronics cleaner in the connectors, just dry and use some spark plug boot grease on the spark plug boots,
And would, after letting any given connector air dry (or help it along w/ some compressed air if available), apply a light coat of dielectric grease before reassembling the connector ..
And would also be checking inside the passenger cabin: if you feel the passenger front floor is wet/damp, then pull down the passenger carpet and inspect the area around the ECM, if wet, then you should fully dry / address the ECM, the ECM connector, and the areas around the ECM (I would use a fan to dry out this area, in addition to spraying / drying the connectors ..
-If- your Sporty has the emissions controls mounted in back of the vehicle / in the gas tank area, then you should also be checking -that- area and drying connectors as well (hence the dry gas tip in the last post),
re: running while poorly - well, it isn't recommended, but if you get a DTC code, I would really recommend purchasing a OBDII scanner, if your Sporty is equipped, and hooking up so you can see real-time values from the ECM & read any codes stored..
If after checking / drying / cleaning the above, if no improvement, then I would move (back) towards the ECM, namely the main engine harness connectors, located (under) the battery on the US model,
If no improvement after that, I would definitely hook up a OBDII scanner to see what is going on w/ the motor & ECM,
And be using a multimeter to check voltage values starting at the engine compartment fuse box -> the voltage values for the sensors mentioned above..
Hope this helps.