Unquiet motor run

Hello.
I haven't found anything about my problem in the search, so my question. My V8 had a too high consumption
17liter before 15liter. That's why I had renewed the distributor caps because they didn't look good anymore. When reading out the fault memory, he showed me short circuit after mass on encoders for coolant temperature G62. So I renewed the giver and eliminated the mass error. After that, the error memory was empty. During the test drive, the V8 ran perfectly and the consumption dropped to 13-14liter according to the indicator. At night I drove on the highway to get it out properly because I noticed that it didn't get to the top speed and when I got slower the engine ran restless as if not all cylinders were running properly. I drove to a parking lot and removed the covers of the ignition cables and saw in the dark that the cables of the left cylinder head were slightly sparking. I read the memory again after that and there was nothing in it. Can the sparks cause him to run so badly? I've never had this problem before, so I'm surprised it happens so suddenly!
Greetings Mark