Well, one method to find out the root cause is:
If it is running bad, try to find out, if it is one cylinder not firing properly.
You can do that, when you measure the temperature of each downpipe,
e.g. with a infrared (laser) thermometer.
Try to measure at the same location on each downpipe.
If one pipe is much colder, that is the cylnder with the problem.
Then you can e.g. swap the injection valve and / or the spark plug cable with that of another cylinder.
If after that, engine is running bad on the other cylinder,
then you know it is a bad injection valve or spark plug cable.
If it is running bad on all cylinders,
there may be many different reasons for it,
check the comments above,
but first try to find if it is one cylinder not running properly.
Just my 2 cents
