My engine has been skipping off and on for the past couple months. Since, then I have changed the plugs/wires and had the o2 sensors cleaned. My mechanic put it on his machine and it's pulling o2 sensor codes, says there's not enough backpressure. I had a custom h pipe installed over a year ago without MIL eliminators. It ran fine up until 2 months ago. My check engine light comes and goes as well which I know is because of the lack of cats . Can anybody tell me how to fix this? Thanks. It's a 98 gt BTW.