355 Question for the experts

Hi All,

Here's the situation:
My 98 355 Spider with 8900 miles on the ticker is acting up a little lately.....I will describe the problem. The car starts immediately, & idles perfectly for a few minutes....then it starts to misfire/skip. The misfire will maintain throughout the rev range...At idle it almost sounds as if two cylinders are firing at the same time... If I let the engine continue to idle the miss will smooth out & then after a minute or so return, etc, etc. The miss sounds equal from both tail pipe outlets. It's not bad, but it does seem like the engine is loading-up slightly. The car is set up with a Tubi & Hyper Flow cats (it had the same symptoms with the old cats.) The check engine light is not coming on for this problem but, I did get DTC codes P0432 & P0422 ( main cat below threshold, bank 2 & 1) P0157 & P0137 ( o2 low sensor voltage bank 2 & bank 1 sensors 2) The car had it's 15,000 service in November 04 including most, if not all of a 30,000 service, timing belts, seals, filters etc.. The only thing that I can think of is that the gas is old (from December 04) & it's down around an 1/8 of a tank. My nearest Ferrari dealership is 3 hours away. Any suggestions will be greatly appreciated. Would like some input before I fuel up and give it an Italian tune up.

TIA, Jati