My money is on the converter, but you could try a tune up of your ignition system.
04 ZX3 RIP [Best Non-SVT FITF12][3/13 ROTM][7/14 ROTM][2014 ROTY][Best Engineered FF14]
01 SE V8 Estate [Organizers Choice FF15][2nd Place MK1 Class FF16]
02 SE SVT Wannabe
02 SVT Future Project