I just purchased a 2009 Ford Flex, it started pausing when I drive it (kinda like a hiccup), i put premium gas in it and it helped, (at first), I took it back and they changed the plugs and core (if that's what they're called...lol) but it stopped but started back up but not as bad. Help, also my RPM hand is not accurate. HELP. any ideas, suggestions would help. Thank you in advance