Last week I put on a new-to-me 3800performance ported m90 s/c and it ran good but I still have this feeling of lack of power. I've had a P0420 from the high flow cat but to be honest have just been ignoring it since it doesn't really hurt anything. Just yesterday I changed the oil and saw a small group of bubbles formed in the bucket of my old oil I also noticed that something just doesn't sound/feel right about the way it drove. On my way home I was going downhill in 35mph and with my foot slightly on the brake pedal the car jerked forward twice and went up about 500 rpms each time. So far it hasn't done this at all when accelerating. When I shut off the car there was a short low pitched hiss. I don't have a scanner tool and its 12:30 in the morning so i couldn't really do much about this now anyways. I am taking a 1/2 day off work so in the morning Ill do a video of the noise and check the vacuum lines/spark plugs and what not. I've got this feeling in my gut though its not gonna be something small.



*edits*
list of mods in "my garage"
72k miles on the car