Stop guessing and look at the readings, this will guide you 99% of the way to the problem.
If the car has ever been stood up for a long time ?, ie months then the injectors can become a bit iffy spray pattern wise, if this is the case injector cleaner will NOT solve the problem, very rarely does it solve problems, despite the claims, proper ultransonic cleaning does though.
To give an example my gf's astra had passed the mot every year without fail, and had perfect emissions, on fast idle, always failed slow idle, did good mpg ran 100% perfect, but whilst having some injectors cleaned I did them as well, the pattern looked very good, and after cleaning had hardly changed, the difference in running though was great, it had lost all the cold engine sluggishness, was smoother, a bit quicker same mpg though, was worth doing.
Post up your results of gas test.
tabetha