Mazda 3 p0420 engine code
 

Mazda3sp23 2003
Hi I have purchased a Mazda have engine light, code p0420 replaced sensor and catalytic converter. Engine light has come back on.
Had a few instances of it cutting out and self restarting prior to sensor and cat change. Havent driven it enough to see if this re occurs, it was suggested the on board computer was flipping out as it had an overload of info.
Not sure on what to do next.
Someone suggested check fuel cap and its sealing.
Ideas?