I have a Jaguar S-Type 2001 with 125K miles.

Lately I've been feeling the car a bit jerky, specially when I'm fully pushing the breaks. If I put it on Neutral, still does it but not as much. Also, when I drive either at low or high speed it feels like its pushing itself all the time (in other words that jerky feeling)

I just did a change of oil in the car so could it be the spark plugs are really bad? I also noticed that the cap for the radiator doesn't close very well, so the pressure might not be where it's supposed to be.

Any suggestions?

Thanks!