I've got a 2000 GT with the same issue. 295,000 miles and still runs good but has that darn 420 code issue. A couple of months ago it started hesitating under acceleration at highway speeds. For example, while cruising at 60+ it would run fine. But if I gave it enough gas to accelerate but not enough to downshift, the engine would start missing and the SES light would flash (random miss). Then, mysteriously, the issue resolved itself. However, a short time later I started having the p0420 issue. Any thoughts guys?
