my 02' f4i sputters when i reach 7.
i recently did a tune up.
changed oil, filter, spark plug.

but even before the tune up it was still missfiring.

what can possibly cause it to sputter?

brother in law told me it could be possibly the
jardine rt-one slip on exhuast and i might need a power commander.

well i need help trying to figure this out.
thanks.