Recently replaced O2 sensor due to p0130 code ; since then code has exhibited intermittent nature ; I've improved and replaced vacuum lines and check valve ; just before code returns engine begins to hesitate on idle slightly. I really don't know what else to try. Awesome ride when ok: 2001 Audi A6 2.8 Quattro V6 w/ 165K miles on it. thanks for any advice .....